A kinetic energy transmission system comprising: a reciprocating actuator; and a direction-conversion and energy output mechanism connected to said reciprocating actuator and comprising: a main housing; an input shaft rotatably mounted in the main housing, having a shaft end of the input shaft connected with the reciprocating actuator for receiving an input reciprocal motion from said reciprocating actuator; an input gear secured on the input shaft; an intermediate gear rotatably mounted in the main housing, and characterized in that said direction-conversion and energy output mechanism comprises: a bidirectional driving gear rotatably mounted about a transmission shaft and engaged with the intermediate gear; the bidirectional driving gear having a plurality of forward-rotation ratchet teeth annularly formed on a first wheel surface of the bidirectional driving gear, and having a plurality of reverse-rotation ratchet teeth annularly formed on a second wheel surface of the bidirectional driving gear opposite to the first wheel surface; a forward-rotation clutch slidably held about the transmission shaft, having a plurality of forward-rotation ratchet teeth annularly formed on a rear wheel surface of the forward-rotation clutch to be normally engaged with the forward-rotation ratchet teeth formed on the first wheel surface of the bidirectional driving gear to be forwardly rotated or driven by the bidirectional driving gear; a front gear secured on the transmission shaft and slidably engaged with the forward-rotation clutch for forwardly rotating the transmission shaft when the forward-rotation clutch is forwardly rotated as forwardly driven by the bidirectional driving gear, and the front gear engaged with an output gear secured on an output shaft for outputting unidirectional rotary motion with rotary kinetic energy; a reverse-rotation clutch rotatably mounted about the transmission shaft and having a plurality of reverse-rotation ratchet teeth annularly formed on a front wheel surface of the reverse-rotation clutch, whereby upon a reverse rotation of the bidirectional driving gear, the reverse-rotation ratchet teeth on the reverse-rotation clutch will be engaged with the reverse-rotation ratchet teeth formed on the second wheel surface of the bidirectional driving gear so as to reversely rotate the reverse-rotation clutch as driven by the bidirectional driving gear; and a reverse-rotation gear set engaged between the reverse-rotation clutch and a rear gear secured on the transmission shaft, whereby upon a reverse rotation of the bidirectional driving gear, the rear gear and the transmission shaft will be forwardly rotated as being engageably driven by the reverse-rotation gear set, thereby also forwardly driving the front gear for outputting unidirectional rotary motion with rotary kinetic energy through the output gear as engaged with the front gear and through the output shaft 21b having the output gear secured on the output shaft; and said transmission shaft and said output shaft respectively rotatably mounted in the main housing. a bidirectional driving gear rotatably mounted about a transmission shaft and engaged with the intermediate gear; the bidirectional driving gear having a plurality of forward-rotation ratchet teeth annularly formed on a first wheel surface of the bidirectional driving gear, and having a plurality of reverse-rotation ratchet teeth annularly formed on a second wheel surface of the bidirectional driving gear opposite to the first wheel surface; a forward-rotation clutch slidably held about the transmission shaft, having a plurality of forward-rotation ratchet teeth annularly formed on a rear wheel surface of the forward-rotation clutch to be normally engaged with the forward-rotation ratchet teeth formed on the first wheel surface of the bidirectional driving gear to be forwardly rotated or driven by the bidirectional driving gear; a front gear secured on the transmission shaft and slidably engaged with the forward-rotation clutch for forwardly rotating the transmission shaft when the forward-rotation clutch is forwardly rotated as forwardly driven by the bidirectional driving gear, and the front gear engaged with an output gear secured on an output shaft for outputting unidirectional rotary motion with rotary kinetic energy; a reverse-rotation clutch rotatably mounted about the transmission shaft and having a plurality of reverse-rotation ratchet teeth annularly formed on a front wheel surface of the reverse-rotation clutch, whereby upon a reverse rotation of the bidirectional driving gear, the reverse-rotation ratchet teeth on the reverse-rotation clutch will be engaged with the reverse-rotation ratchet teeth formed on the second wheel surface of the bidirectional driving gear so as to reversely rotate the reverse-rotation clutch as driven by the bidirectional driving gear; and a reverse-rotation gear set engaged between the reverse-rotation clutch and a rear gear secured on the transmission shaft, whereby upon a reverse rotation of the bidirectional driving gear, the rear gear and the transmission shaft will be forwardly rotated as being engageably driven by the reverse-rotation gear set, thereby also forwardly driving the front gear for outputting unidirectional rotary motion with rotary kinetic energy through the output gear as engaged with the front gear and through the output shaft 21b having the output gear secured on the output shaft; and said transmission shaft and said output shaft respectively rotatably mounted in the main housing.