The invention discloses a power transmission system of a self-propelled mowing car, which comprises a running mechanism and a mowing mechanism which are driven by the same upright shaft engine, and ischaracterized in that: the upright shaft engine drives a duplex pulley (1) to drive the running mechanism and the mowing mechanism respectively; the pulley which drives the running mechanism drives astepless change wheel (2) to drive a large pulley (3) on a driving axle (8); a large chain wheel (12) is driven by a small chain wheel to drive a main deceleration pinion (11) and a main large deceleration gear (16); two half shafts are driven by a differential gear (15) for driving two tyres to turn; the pulley which drives the mowing mechanism drives a pulley (4), the pulley (4), a left cuttingknife (6) and a toothed pulley are arranged on the same shaft to rotate together; in the same way, a right cutting knife (7) is provided with a toothed pulley (5) coaxially; the toothed pulley of theleft cutting knife (6) drives the toothed pulley (5) of the right cutting knife (7) and the right cutting knife (7) through a cog belt and a tension wheel; and the rotation directions of two cuttingknives are opposite;the tow cutting knives rotate in opposite directions and forms a right angle.The power transmission system of the self-propelled mowing car has the advantages of convenient operation, high efficiency, reliability and energy conservation.