The invention discloses a mower belt transmission mechanism which is compact in structure and stable in transmission. The mechanism comprises a driving belt wheel, a driven belt wheel, a driving arm and a reciprocating part. The driving belt wheel is in transmission connection with a horizontal power output shaft, the axis of the driving belt wheel coincides with the axis of the power output shaft, a wheel disc of the driven belt wheel is arranged horizontally or slantwise towards the driving belt wheel, transmission between the driving belt wheel and the driven belt wheel is performed through a belt, the driving arm is arranged eccentrically on the driven belt wheel, an accommodation groove is formed in the reciprocating part, and the driving arm is disposed in the accommodation groove and can drive the reciprocating part to reciprocate horizontally, and one end of the reciprocating part is fixed in the middle of a blade fixing plate which is fixed in the middle of an upper movable tool.