The invention belongs to the technical field of new energy vehicle power assembly components, and relates to a single motor hybrid power system based on a stepless transmission. The system comprises an engine, a stepless transmission device, a planetary gear device and a motor; the stepless transmission device comprises an active belt wheel group, a metal belt and a driven belt wheel group; the stepless transmission device and the planetary gear device are connected through a first clutch and a braking device; the planetary gear device comprises a gear ring, a planetary gear, a planetary frameand a solar gear. The hybrid power system is provided with three torque coupling type working modes, and fuel economy and driving acceleration performance under different driving working conditions can be improved; the hybrid power system is provided with two pure electric modes which can improve the working efficiency under low speed and high speed working conditions of the motor respectively, and therefore a motor with a smaller size can be adopted; the hybrid power system is provided with a power back flowing type working mode, so that the transmission ratio range is wider compared with aP2 parallel type hybrid power vehicle, and the application range of the driving condition is wider.