Electric vehicle charging technology is one of the important technologies of electric vehicle application. The LLC resonant converter is characterized by high efficiency, wide output voltage range, high power, etc., and is widely used in electric vehicle chargers. Due to the complexity of the resonant process analysis, the LLC resonant converter is usually designed using the First Harmonic Method (FHA) method. This method does not take into account Discontinuous Conduction Mode (DCM), thereby resulting in a large error, and needs repeated iteration to find the appropriate circuit parameters. This paper presents a parameter optimization design method based on DCM analysis, the gain of the converter circuit can be solved more precisely, and the design process does not require iteration. A 3 300 W LLC converter simulation model with 250~430 V outputs and 400 V inputs is built, results show that the LLC resonant converter parameters based on DCM analysis possesses high operation efficiency, meets the converter's voltage gain requirements, and reduces the voltage gain error by 74.9% than the traditional method. This work is supported by National Key Research and Development Program of China (No. 2018YFB1503001), Technical Standard Project of Shanghai Science and Technology Commission (No. 18DZ2205700), and Industry- University-Research Cooperation Program of Minhang (No. 2017MH271).