Current digital simulation system used in relay protection can only achieve the secondary side protection simulation. Considering the defect, a primary and secondary combined digital simulation system of distribution network relay protection is put forward. Overall design scheme is given, the key technologies and related realizing methods are introduced from the aspects of fault data obtaining, external data interface, fault data conversion and protection algorithm. The proposed simulation system based on PSCAD/EMTDE and VC++ is constructed, and an over-current protection simulation example is performed. The simulation results are consistent with theoretical analysis, which verify the correctness and effectiveness of the simulation system.