This paper describes the similarities and differences between the DC bias’ influences on transformers made by geomagnetic induced current (GIC) and HVDC in detail. Then the simulation of GIC is discussed and a simulation system is built based on PSCAD / EMTDC platform. Then the case of post-fault over load is simulated and the performance of distance protection is analyzed. The result demonstrates that when GIC invades power systems through transformers with neutral grounded the transformer core may become asymmetric saturated, which may cause the measured impedance in the state of cyclical fluctuations and distance protection zone Ⅲ which backs up for adjacent transformer is possible to mal-operate under the case of post-fault over load.