In view of the problem that the frequency injection method used in current DC system grounding fault detection will bring large ripples to the DC system and is greatly influenced by the distributed capacitance, this paper proposes a method to increase the detection precision by adding the AC power to the positive and negative DC buses and adding the short circuit capacitance path on the bus.That is the leakage current of the fault branch is obtained according to the two frequency conversion injection of AC signals,two two element equations are established to accurately calculate the earth resistance under the premise of avoiding the influence of distributed capacitance.This method can input a large capacitance branch in the bus bar, which can effectively avoid the error caused by the busbar distributed capacitance and meet the requirements of the State Grid.,and the small signal injected into the system is within the specified range of ripple,the influence of branch distribution capacitance is also avoided.The method effectively reduces the error caused by the sensor measurement precision and the distributed capacitance on the DC system insulation detection, reduces the false alarm probability of the insulation detection equipment, improves the detection precision and uses the experimental simulation to verify the feasibility of the method.