One kind of high voltage DC power supply applied for electron linear accelerator is described in the paper. The median frequency AC voltage is supplied by a frequency converter. The DC high voltage is realized by resonant circuit, air core transformer and multi-winding double-rectifying circuit. The equipment adopts air core transformer instead of high-frequency transformer, which solves the difficult design problem of high-frequency transformer. And the power supply is with high power factor, reliability and low loss. In addition the simulation and experiments are described in the paper.