As a key component of half bridge LLC resonant converter,high frequency transformer design may affect loss,stability of output voltage and heat value of switch power supply.Therefore,this paper aims at design for 5 kW high frequency transformer,introduces type selection principles for high frequency transformer cores and takes area product method to determine types of winding wires and cores.By further optimization calculation it gets numbers of windings,transformer loss and other relevant parameters.Final high frequency transformer parameters include that type of former vice side winding wire is No.18,type of core is EE-100,transformer ratio is 0.7375,former vice side winding numbers are 59 and 80 respectively and transformer loss is 16.187 W.