LLC converter has the soft switching characteristies of zero voltage switching of primary side and zero current switching of secondary side. It has high power density, which is widely used in vehicle power supply, LED and other fields. However, due to the complexity of LLC resonant converter, there is a lack of an ffective and clear design method for resonant parameters. Based on the first harmonic approximation ( FHA) , the traditional gain formula is further simplified, and a simple and clear resonant parameter design method is proposed in this paper. In order to verify the optimal parameters of this method, a 7.5 kW experimental prototype is made. The output voltage range is 100 V ~375 V, and the constant power output of 7.5 kW can be achieved in the voltage range of 200 V ~ 375 V. the average efficiency of the whole machine can reach more than 95% , and the peak efficiency can reach 96. 7%. The experimental results show that the proposed parameter design method can maximize the efficiency of the converter.