The invention provides an elevator space overload detecting method. The elevator space overload detecting method comprises the steps of: S1) setting an optical sensor in an elevator car; S2) detecting the illumination brightness on the top surface of the car to obtain a first illumination brightness, and detecting the illumination brightness on the bottom surface of the car to obtain a second illumination brightness; S3) calculating an elevator space occupancy rate according to the difference between the first illumination brightness and the second illumination brightness; and S4) judging if an elevator overloads according to the space occupancy rate, and controlling the start and the stop of an elevator motor. Through the mode, the elevator space overload detecting method can intelligently judge if the elevator space can be used, and can save the waiting time of other passengers when objects with large sizes and low weights occupy the space.