This paper mainly designs a bidirectional LLC resonant converter with symmetrical structure about the analysis of theory and optimization design of parameters to reduce the loss of power switching. Firstly, the operating principle of the converter is analyzed by the approach of fundamental component method(FHA) to obtain the DC voltage gain versus. The paper also analyses the impaction of load and parasitic parameters on ZVS and ZCS of power switches. A reasonable quality factor and resonant parameters are designed to ensure that the converter will not enter the capacitive working area. Finally, the simulation model of bidirectional LLC resonant converter is built in the saber simulation software, and the feasibility and accuracy of the parameter optimization design are verified by the simulation results and theoretical analysis.