An automatic transmission, wherein the automatic transmission comprises: a first left-contrate-tooth combined fluted disc, a first right-contrate-tooth combined fluted disc and a two stage transmission ordinary gear train; and the two stage transmission ordinary gear train comprises: a transmission input shaft, a transmission intermediate shaft, a transmission output shaft, a first external-gear right-contrate-tooth combined gear, a first external-gear left-contrate-tooth combined gear, a first gear and a second gear; wherein, the transmission input shaft, the transmission intermediate shaft and the transmission output shaft are arranged in parallel, the first external-gear right-contrate-tooth combined gear is fixedly connected to the transmission input shaft by a key, the first external-gear right-contrate-tooth combined gear and the first gear engage by peripheral teeth, and the first gear is fixedly connected to the transmission intermediate shaft by a key; the second gear is fixedly connected to the transmission output shaft by a key, the second gear and the first external-gear left-contrate-tooth combined gear engage by peripheral teeth, and the first external-gear left-contrate-tooth combined gear is loosely nested on the transmission intermediate shaft; the first left-contrate-tooth combined fluted disc is able to turn synchronously with the transmission output shaft, and is able to axially move along the transmission output shaft; the first right-contrate-tooth combined fluted disc is able to turn synchronously with the transmission intermediate shaft, and is able to axially move along the transmission intermediate shaft; the first external-gear right-contrate-tooth combined gear further has right contrate teeth; after the first left-contrate-tooth combined fluted disc moves to the left to a specified position, the right contrate teeth of the first external-gear right-contrate-tooth combined gear and the left contrate teeth of the first left-contrate-tooth combined fluted disc engage; the first external-gear left-contrate-tooth combined gear further has left contrate teeth; after the first right-contrate-tooth combined fluted disc moves to the right to a specified position, the left contrate teeth of the first external-gear left-contrate-tooth combined gear and the right contrate teeth of the first right-contrate-tooth combined fluted disc engage; and when the first left-contrate-tooth combined fluted disc and the first right-contrate-tooth combined fluted disc are in an intermediate position, the first left-contrate-tooth combined fluted disc does not engage with the right contrate teeth of the first external-gear right-contrate-tooth combined gear, and the first right-contrate-tooth combined fluted disc does not engage with the left contrate teeth of the first external-gear left-contrate-tooth combined gear. a first left-contrate-tooth combined fluted disc, a first right-contrate-tooth combined fluted disc and a two stage transmission ordinary gear train; and the two stage transmission ordinary gear train comprises: a transmission input shaft, a transmission intermediate shaft, a transmission output shaft, a first external-gear right-contrate-tooth combined gear, a first external-gear left-contrate-tooth combined gear, a first gear and a second gear; wherein, the transmission input shaft, the transmission intermediate shaft and the transmission output shaft are arranged in parallel, the first external-gear right-contrate-tooth combined gear is fixedly connected to the transmission input shaft by a key, the first external-gear right-contrate-tooth combined gear and the first gear engage by peripheral teeth, and the first gear is fixedly connected to the transmission intermediate shaft by a key; the second gear is fixedly connected to the transmission output shaft by a key, the second gear and the first external-gear left-contrate-tooth combined gear engage by peripheral teeth, and the first external-gear left-contrate-tooth combined gear is loosely nested on the transmission intermediate shaft; the first left-contrate-tooth combined fluted disc is able to turn synchronously with the transmission output shaft, and is able to axially move along the transmission output shaft; the first right-contrate-tooth combined fluted disc is able to turn synchronously with the transmission intermediate shaft, and is able to axially move along the transmission intermediate shaft; the first external-gear right-contrate-tooth combined gear further has right contrate teeth; after the first left-contrate-tooth combined fluted disc moves to the left to a specified position, the right contrate teeth of the first external-gear right-contrate-tooth combined gear and the left contrate teeth of the first left-contrate-tooth combined fluted disc engage; the first external-gear left-contrate-tooth combined gear further has left contrate teeth; after the first right-contrate-tooth combined fluted disc moves to the right to a specified position, the left contrate teeth of the first external-gear left-contrate-tooth combined gear and the right contrate teeth of the first right-contrate-tooth combined fluted disc engage; and when the first left-contrate-tooth combined fluted disc and the first right-contrate-tooth combined fluted disc are in an intermediate position, the first left-contrate-tooth combined fluted disc does not engage with the right contrate teeth of the first external-gear right-contrate-tooth combined gear, and the first right-contrate-tooth combined fluted disc does not engage with the left contrate teeth of the first external-gear left-contrate-tooth combined gear.