3D resistivity inversion under the condition of undulate topography is a difficult problem in the current resistivity method. In order to solve this problem, the authors use a weighted regularized conjugate gradient method to accomplish three-dimensional resistivity inversion algorithm of undulating terrain. The method introduces weighted regularization thought which can significantly reduce the problem of the objective function iteration divergence and improve the stability of the inversion. A comparison of the two methods in eliminating topographic influence in the inversion shows that the direct use of the terrain resistivity method of 3D inversion can yield better resolution, which can effectively eliminate the error caused by terrain. Nevertheless, under the condition that the rolling angle is almost vertical such as the cases of rivers and dams. The continual utilization of this method can hardly get a satisfactory result because it causes the inverse divergence. In such cases, the use of topographic correction method has some advantageous effects.