Abstract: A fully mechanized mining face tend to show great change in coal thickness, which affects the safe, efficient coal mining. Therefore, it is necessary to detect the change in the coal thickness before stoping. To determine the response of electromagnetic (EM) wave penetration to the change in the coal thickness, this study established a three-dimensional model of fully mechanized mining faces using simulation software. On this basis, this study simulated the change in the coal thickness under different ratios of the coal-to-rock resistivity based on EM wave penetration. The results are as follows: (1) With a decrease in the coal thickness, the intensity of the EM field corresponding to different ratios of the coal-to-rock resistivity decreased in the form of a parabola. This result indicates that a smaller coal thickness is associated with a poorer EM wave penetration ability; (2) For the same coal thickness, a greater ratio of coal-to-rock resistivity corresponded to a greater intensity of the EM field and a greater distance of EM wave penetration; (3) For the mining faces with a coal thickness of less than 8 m, the intensity of the EM field changed at a high rate, indicating that the change in the coal thickness significantly changed the intensity of the EM field. Therefore, it is feasible to interpret the change in geological anomalies based only on the relative change in the coal thickness in this case; (4) For the mining faces with a coal thickness greater than 8 m, the intensity of the EM field changed at a low rate, indicating that the change in coal thickness could not significantly change in the intensity of the EM field. Therefore, the geological anomalous areas cannot be interpreted based only on the relative change in the coal thickness in this case. Instead, it is necessary to delineate the geological anomalous areas by determining the appropriate thinning rate of coal based on the changing rate of the intensity of the EM field, as well as the ratio of coal-to-rock resistivity and the normal coal seam thickness. As shown by the detection results of the No.1305 mining face in the Shaanxi Jinyuan Zhaoxian Mining Co., Ltd., the intensity of the EM field changes at an average rate of 1.2338 dB/m as the coal seam thickness of the mining face decreases from 16.4 m to 11.2 m. This result reflects that intensity of the EM field of a mining face with ultra-thick coal seams decreases slowly with a decrease in the coal thickness. As indicated by the detection results of the No.1610A mining face in the Zhangji Coal Mine of Huaihe Mining (Group) Co., Ltd., the intensity of the EM field changes at an average rate of 3.7038 dB/m as the coal seam thickness of the mining face decreases from 5.8 m to 2.0 m. This result reflects that intensity of the EM field of a mining face decreases quickly with a decrease in the coal thickness. The results of this study can be used to reasonably identify the range of thin coal areas and the degree of the coal seam thinning in these areas and to reliably delineate geological anomalous areas.