Impedance matching is a common issue in wireless power transfer system. This paper focuses a novel viewpoint on DC-DC converter for resistance conversion in rectifying circuit. We give a theoretical analysis on the input/output resistance relation and voltage relation of three topologies DC-DC converters both in continuous conduction mode (CCM) and discontinuous conduction mode (DCM). The input resistance of buck-boost converter in DCM is independent of the input voltage and load resistance. We succeed in applying this buck-boost converter in microwave power transfer system for impedance matching.