The invention discloses an electric vehicle transmission and a speed changing method thereof, comprising a hydraulic torque converter with a locking function, a planetary gear mechanism, an oil pump, an oil pump motor, a controller, a speed reduction mechanism and a differential. The hydraulic torque converter with lock-up function includes a pump wheel, a turbine, an input shaft, an output shaft, etc., the drive motor is connected to the pump wheel through the input shaft, and the turbine is connected to the output shaft. The torque on the output shaft is transmitted to the planetary gear mechanism. The planetary gear mechanism includes sun gear, planet carrier, large ring gear, clutch, brake and other parts. The torque is output from the planet carrier through the planetary gear mechanism, and drives the vehicle to travel after passing through the reduction mechanism and the differential. The transmission can realize two mechanical transmission gears and one hydraulic torque conversion gear. The oil pump is used to provide pressure to the transmission hydraulic system. The electric vehicle transmission of the invention can improve the climbing performance and acceleration performance of the electric vehicle, and at the same time enhance the reliability of the vehicle.