This paper analyzes the necessity of establishing the dc bus voltage of the Dual Active Bridge (Dual Active Bridge, DAB) converter when starting up. And the startup method that the DC-bus voltage is built up only by high-voltage-side body diode of the DAB converter rectifying is proposed in the paper. The detailed steps and the software realization method are elaborated. The saturation phenomenon of the inductor and the current surge can be effectively avoided in this way and the method presented in this paper is implemented only by digital control, so this method does not need to add extra hardware. Finally, the experimental results verify the method.