This application provides an electric motor control method and device, a system, and an electric vehicle. The method includes: obtaining an input power and an output power of an electric motor; calculating a loss power based on the input power and the output power; determining a target d-axis current of the electric motor, where the target d-axis current is calculated by a calculation module based on the loss power, the calculation module includes a tracking differentiator, and the calculation module operates in a controller; and determining a duty ratio based on the target d-axis current and a preset q-axis current, and inputting the duty ratio into an inverter so that the inverter drives, based on the duty ratio, the electric motor to operate.