Based on the wireless charging system controlled by the secondary side, a control strategy for the stable operation of the wireless charging system without the communication module between primary side and secondary side was proposed. First, the work stability of the series resonance wireless charging system is analyzed, and in order to ensure stable operation, the secondary side needs to control the output voltage, control output voltage by adjusting the cycle of duty of semi-controlled rectifier circuit . Further analysis shows that the system may not work properly under the two conditions of reduced load and increased secondary output voltage. By introducing the method of estimating the load and the duty cycle of the secondary switch on the primary side, the primary side can synchronously adjust the input voltage without the communication module, and the system can work stably. Finally, the simulation of the control strategy is verified in MATLAB.