In order to study the DC bias mechanism of AC power grid caused by metro traction system, this paper takes the ground potential as the object, and carries out time-domain simulation about the influence of metro traction system on the adjacent ground potential. Firstly, the time domain simulation model of ground potential including rail, rail-ground transition resistance, tunnel, structural reinforcement and drainage network is established. Then, the time domain waveforms of ground potential under different conditions are simulated which include metro departure, metro arrival, metros running in the same direction and metros running in the opposite direction. Results show that the time variation characteristics of DC bias current and surface potential originate from the time variation of traction current and stray current, and the magnitude, polarity and peak time of surface potential are affected by the relative position among metro, observation location, and traction station; When multiple metros are considered, the superposition effect of ground potential generated by each metro decreases as the running interval increases, and the time domain waveform of ground potential shows intermittent oscillation. On the contrary, the superposition effect of ground potential generated by each train increases when the running interval decreases leading to continuous oscillation of ground potential. Moreover, large pulses of surface potential may appear if two metros enter or leave the station simultaneously. In conclusion, the time-varying traction current, train position, operation interval, quantity, and operation state of metros are the main causes for the change of polarity, magnitude, and oscillation frequency of DC bias current and potential. Finally, the simulation model and results are verified by measurement.