The purpose of the utility model is to provide a hydraulic motor speed reducer. According to the speed reducer, technical problems of small transmission ratio scope, large and ponderous size, low efficiency and reliability, short life, large operation noise and small transmission torque of conventional two-stage common cylindrical-gear speed reducers are solved. A hydraulic motor main shaft is fixedly connected with a sun gear, the sun gear is meshed with planetary gears, so that first-stage deceleration is realized. A bearing-assembly front end is fixedly connected with the planetary gears, a bearing assembly is arranged on a first cycloid wheel and a second cycloid wheel in a matching manner, and the first cycloid wheel and the second cycloid wheel are alternatively meshed with a needle pin. The needle pin is embedded on a housing inner wall of the speed reducer, so that second-stage deceleration is formed. The hydraulic motor speed reducer provided by the utility model adopts a first-stage straight-teeth planetary transmission mechanism and a second-stage cycloid-wheel tooth-difference transmission mechanism to increase a transmission ratio and raise transmission torque. The speed reducer has the advantages of compact structure, high efficiency, stable rotation, low noise, reliable operation and long service life.