The invention relates to a half load detection method and system, and a washing device. The half load detection method comprises the following steps that firstly, a half load judgment linear equation of a motor is established; and secondly, the current working state, namely half load or full load, of the motor is judged according to the half load judgment linear equation, and the rotation speed of the motor is controlled according to the load condition. Through the half load detection method and detection system, by establishing the half load judgment linear equation, the full load state and the half load state are detected and judged through linear regulations of the half load judgment linear equation, the judgment accuracy is improved, and the false detection or missing detection phenomenon is reduced; and by adjusting the rotation speed of the motor according to the detection results, noise generated by the motor under the half load condition can be effectively lowered.