With the rapid development of HVDC transmission in China, the earth return current during monopolar operation of HVDC system have caused serious electromagnetic compatibility problems. From a general aspect, the numerical computation method of wide-area current field considering the deep earth resistivity is presented, and a complex image method based on self-adaptive sampling is proposed to compute the Green’s function with complex horizontal multi-layer ground accurately. The new and valuable concepts given in this paper can provide a useful reference for analyzing the electromagnetic compatibility problems caused by the earth return current of HVDC transmission.