At present, in the process of grouting mine-out area in coal mine, there exist some practical problems, such as leakage of slurry, which endangers engineering safety. Monitoring the diffusion range of grout is an important research direction in coal mining. TEM is a time-domain electrical exploration method, which has the characteristics of high electrical resolution, strong environmental adaptability, strong penetration of high resistance layer, small interference effect and high working efficiency. It has good exploration effect in mined-out area and karst. By TEM, the electrical property and scale of the underground geological body are determined according to the analysis of its attenuation characteristics. Therefore, the Caojiatan mine goaf project used TEM to analyze and compare the exploration results before and after grouting, conformed the downstream and upstream diffusion ranges of grout are 150 m and 100 m, respectively, and the height of grout accumulation is 1005 m. It provides a powerful guarantee for the safety production of coal mines.