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Analysis and calculation of different ground electrical parameter variation caused by surface soil moisture are conducted, and the impact of the variation on the propagation of the electromagnetic field of lightning return strokes is examined. A two dimension fraction Brown motion model is presented for describing the natural rough surface. The adopted method is based on the Cooray and Wait approximate calculation, taking advantage of the soil electrical parameter equivalent model put forward by Scott. It is shown that the impact of surface humidity on lightning electromagnetic field propagation can not be ignored and the impact on the horizontal electric field is larger than that on the vertical electric field; the peak value of the horizontal electric field is inversely proportional to the soil moisture. With the increasing humidity, the increase of both vertical and horizontal electric fields deceases with time, but the peak values hardly change. Therefore, the impact of soil moisture must be considered when studying lightning electromagnetic radiation environments because the rainfall always accompanies with thunderstorms.