A motor demagnetization detection method includes the following steps. The speed of a motor is calculated according to the rotation angle of the motor. When determining that the motor respectively maintains a first and second steady state according to the speed, the three-phase current values of the motor are received to obtain first and second steady-state data. An equivalent magnetic flux is calculated according to the first and second steady-state data. The motor is repeatedly driven to maintain the first and second steady states, the first and second steady-state data are updated according to the three-phase current values, and the equivalent magnetic flux is again calculated to generate equivalent magnetic fluxes. A magnetic flux change is calculated according to equivalent magnetic fluxes. A demagnetization warning is issued according to a comparison of the magnetic flux change and a demagnetization warning value.