In view of the LLC series resonant converter topology, it is well known as suitable for small and medium power application, the stabilization of output voltage and the environment of step-dowm. The scheme is proposed for a kind of the step-up DC/DC transformer based on LLC resonant full bridge. The detailed design method is given. The 3KW prototype is built. The experimental results show it has the characteristics of the constant current discharge, wide working frequency and high power desity, as well, the feasibility is verified.