In substation DC system, the relay misoperation caused by ground fault may lead to hidden danger for the power system. Based on computer HILS(hardware in-the-loop simulation ) technology, the authors built a platform with eMEGAsim Real-time Simulator to analyze the relay misoperation of DC system. In the platform, through the substitution principle, the relay model block in the RT-LAB software is replaced by a real object, and the whole electrical characteristics of original simulation model kept unchanged, which could realize the effective connection between the mathematical model and the engineering object. Using the platform, the authors analyze the influences of different Resistance values and the system cable capacitance on the relay misoperation. Compared with off-line simulation, the HILS platform can effectively improve the speed of simulation analysis, show the real switching action process, and the analysis results have reference significance to the actual debugging.