The DC power supply based on LLC resonant converter has the advantages of a wide range of output voltage, high efficiency, small size and so on. This paper, firstly, analyzes the mathematical model of the three level half bridge LLC resonant converter. On this basis, the Pulse Frequency Modulation (PFM) control system which uses Digital Signal Processor (DSP) as the core is designed in the loop frequency modulation process, the loading times of cycle value and comparative value are not consistent, resulting in abnormal vibration of the circuit. In order to solve the problem, a method of assign cycle value and comparative value in the specific area is proposed. Finally, the result from the experiment shows that this method is effective. This work is supported by National Natural Science Foundation of China (No. 61503122 and No. 61803145).