The operation experience shows that the root cause of the harm of large-scale DC magnetic bias caused by DC grounding electrode lies in the uneven surface potential distribution. The Chebyshev polynomial is used to fit the Hankel transform kernel function of surface potential in order to solve the problem of solving the surface potential distribution in the complex earth model of wide area depth stratification. Through the shift operation, coefficient expansion and truncation error determination of Chebyshev polynomial, the adaptive order fitting method of Chebyshev polynomial for kernel function is obtained, which greatly reduces the calculation difficulty of surface potential distribution in a large area caused by DC grounding electrode. Compared with the standard grounding calculation software CDEGS, the earth surface potential deviation is less than 1 V in the range of 1 km ~ 100 km when the DC grounding current is 5 000 A. The influence of the Chebyshev polynomial series on the results is further analyzed, and it is confirmed that the accuracy of the 20-order Chebyshev polynomial can meet the application requirements of the general DC bias risk assessment. The surface potential rapid assessment method based on the shifted Chebyshev polynomial provides a basic technical means for the risk assessment of DC bias, which is helpful to reduce the difficulty of risk assessment of DC bias in power grid.