In IT system, It will cause the neutral-point voltage of rectifier unbalance when conduction voltage of diode changes or the DC side line-to-ground insulation resistance decline asymmetrily,then DC current will flows into the neutral point of the AC system through the earth, which makes IMD that based on DC injection false or missing alarming. Through analysis, the neutral-point voltage to ground of rectifier can be equivalent to a Thevenin model ,Which can act on the DC impedance network of the original system,then derive the mechanism of neutral-point voltage on IMD,meanwhile,the idea of change injection signal direction are present,record the insulation resistance value in forward and reverse way,the real insulation value can be calculated after processing the record.Finally,experiment results validate the proposed scheme.