Abstract: In the high frequency isolated grid connected system,LLC resonant converter can significantly improve the overall efficiency of the system when it works at the resonant point. However,under the actual working conditions,it is difficult to ensure that the working frequency is the same as the resonant frequency. In this paper,in order to track the resonant frequency points,an adaptive frequency tracking strategy is proposed to distinguish the working modes of the resonant converter by the working state of the secondary rectifier diode of the transformer. Finally,a prototype of 1 kW LLC resonant converter has been designed and the effectiveness of the frequency tracking control strategy is verified by experiments.