Aiming at the problem that the short circuit current rises rapidly and the breaker cannot move quickly and cutting off the fault in the DC distribution network when caused short circuit fault .In this paper, a simulation model is built based on case that the to convert AC to DC power by a three-phase two level voltage source converter (VSC) and supply power to passive network. The fault characteristics of the short circuit fault and mono-polar grounding fault was analyzed, a protection scheme united the under voltage protection and current differential protection was proved and improved power supply reliability by switching back power supply. Finally, the simulation verification is carried out on the PSCAD/EMTDC platform. The results show that the protection scheme can act quickly and reliable when short circuit fault occurs in DC distribution network, and has good selectivity and sensitivity.