Through the mechanism analysis of protective relay misoperation because of one point earthing in power DC system, this paper proposes safety assessment measures based on system capacitance analysis. Combined with conditions of protective relay misoperation caused by one point earthing and State Grid eighteen anti accident measures, it proposes safety design measures that control negative voltage to earth in DC insulation monitoring system, mainly includes the single arm detection bridge and temporary fixed bridge, and describes the specific implementation plan and corresponding algorithm. According to these measures, a higher safety DC insulation monitoring system is designed which can prevent protective relay misoperation because of one point earthing.