A crank structure of a bicycle, the crank structure comprising: a crank mechanism, comprising a crank, a shaft end gear disposed on one end of the crank and a treadle end gear disposed on another end of the crank; a transmission mechanism, disposed in the crank; and a rotation arm unit, comprising a first rotation arm and a second rotation arm, wherein two ends of the second rotation arm are respectively connected to one end of the first rotation arm and the treadle end gear; characterized in that: the transmission mechanism comprises a rotation shaft and a first gear and a second gear connected to two ends of the rotation shaft, wherein the first gear and the shaft end gear are engaged for transmission, and the second gear and the treadle end gear are engaged for transmission; wherein the transmission mechanism further comprises a connection shaft sleeve, the rotation shaft comprises a first rotation shaft and a second rotation shaft, the first gear is connected to one end of the first rotation shaft, the second gear is connected to one end of the second rotation shaft, the connection shaft sleeve is adapted to sheathe another end of the first rotation shaft and another end of the second rotation shaft and is rotatable with the first rotation shaft and the second rotation shaft. a crank mechanism, comprising a crank, a shaft end gear disposed on one end of the crank and a treadle end gear disposed on another end of the crank; a transmission mechanism, disposed in the crank; and a rotation arm unit, comprising a first rotation arm and a second rotation arm, wherein two ends of the second rotation arm are respectively connected to one end of the first rotation arm and the treadle end gear; characterized in that: the transmission mechanism comprises a rotation shaft and a first gear and a second gear connected to two ends of the rotation shaft, wherein the first gear and the shaft end gear are engaged for transmission, and the second gear and the treadle end gear are engaged for transmission; wherein the transmission mechanism further comprises a connection shaft sleeve, the rotation shaft comprises a first rotation shaft and a second rotation shaft, the first gear is connected to one end of the first rotation shaft, the second gear is connected to one end of the second rotation shaft, the connection shaft sleeve is adapted to sheathe another end of the first rotation shaft and another end of the second rotation shaft and is rotatable with the first rotation shaft and the second rotation shaft. the transmission mechanism comprises a rotation shaft and a first gear and a second gear connected to two ends of the rotation shaft, wherein the first gear and the shaft end gear are engaged for transmission, and the second gear and the treadle end gear are engaged for transmission; wherein the transmission mechanism further comprises a connection shaft sleeve, the rotation shaft comprises a first rotation shaft and a second rotation shaft, the first gear is connected to one end of the first rotation shaft, the second gear is connected to one end of the second rotation shaft, the connection shaft sleeve is adapted to sheathe another end of the first rotation shaft and another end of the second rotation shaft and is rotatable with the first rotation shaft and the second rotation shaft.