A parameter design method for a full bridge LLC resonant converter is proposed, which includes magnetic inductance, resonant inductance, resonant capacitance and transformer primary to secondary side turn ratio. Through this design method, zero voltage switching (ZVS) of the primary side switching tube can be achieved effectively while obtaining a larger magnetic inductance,which means the reduced losses of the switching devices and higher converter efficiency. Moreover, the proposed method of drawing voltage gain characteristic curve clusters can intuitively and efficiently determine the inductance coefficient K and quality factor Q, optimizing parameter design. Meanwhile, theoretical analysis and experimental research are conducted on the influence of inductance coefficient K on the efficiency of the converter. Finally, the correctness and validity of this parameter design method and related theoretical research are verified by the 200 W experimental results of on a full bridge LLC prototype.