DC magnetic bias current will be caused in transformers when HVDC systems operating in mono-pole situation. The combined influence on DC magnetic bias of 500 kV transformers is researched, taking ±800 kV Jinbei-Nanjing, Ximeng-Taizhou and Longbin-Xuzhou in northern Jiangsu for example. A comprehensive horizontal multilayer soil model is adopted, considering different soil conditions adjacent to different ground electrodes, based on which the DC system model of northern Jiangsu is built. The DC magnetic bias currents in substations nearby are analyzed when one or more in three HVDC power transmission lines are operated in monopole operation mode. Simulation results show that the greater the soil resistivity, the higher the earth surface potential; when multi HVDC lines are operated in monopole mode simultaneously, the value of DC current is equal to algebraic sum of the DC current of each HVDC line solely operated in monopole mode; the restraining measures of adjacent transformers should be considered comprehensively.