本文利用不平衡电桥法给出了一个较为精确的，用于电网直流系统计算正负母线绝缘值都出现降低的情况下的正、负绝缘电阻的公式。同时提出了一种主—从式的绝缘监测小系统的设计方案。即用一个绝缘主模块来测量主回路的绝缘电阻，而用专门的支路模块来判断出现绝缘降低的那条支路，并且在实际应用中取得了很好的效果。关键词：不平衡电桥；主—从式； 绝缘监测。Abstract：By unbalant electricity bridge ,this article educed a accurate formulation with which the insulation resistance can be calculated when both the plus insulation resistance and the minus insulation resistance decend in DC system. Moreover it bring forward a design project of small insulation monitoring system named master-slave type.Namely measured insulation resistance of main road with main insulation module,and measured which road’ insulation resistance decended with branched insulation module.And obtained good result in use.Key Words：Unbalant electricity bridge； Master-slave type； Insulation detecting.