The invention discloses a driving mechanism for a bicycle or a tricycle. A horizontally-arranged left half axle and a right half axle are each rotationally supported on a frame through a bearing, and the axis of the left half axle and the axis of the right half axle are located on the same straight line. A left pedal is fixedly arranged at the left end of the left half axle, the middle portion of the left half axle is sleeved with a left chain wheel through a one-way bearing or a one-way ratchet wheel, and the right end of the left half axle is coaxially and fixedly sleeved with a left bevel gear. The left end of the right half axle is coaxially and fixedly sleeved with a right bevel gear, the middle portion of the right half axle is sleeved with a right chain wheel through a one-way bearing or a one-way ratchet wheel, and a right pedal is fixedly arranged at the right end of the right half axle. The left bevel gear and the right bevel gear can be selectively engaged with or disengaged from the same middle transition bevel gear. The front end of a sliding shaft is fixedly sleeved with the middle transition bevel gear, and the sliding shaft is in sliding fit with an inner hole of a locating sleeve. By means of the driving mechanism, a one-way transmission manner and a common transmission manner can be flexibly switched, and the included angle between the left pedal and the right pedal can be conveniently adjusted according to the height and the leg length of a rider.