Based on the full-space model, the data processing method of underground transient electromagnetic method was studied. The full-space earth-electricity model was established according to the measured coal seam data, and the full-space transient electromagnetic response of the water filled goaf was simulated by the time domain finite difference method. The results showed that the low resistance area of the apparent resistivity contour line could accurately reflect the water abundance situation in the goaf. The underground transient electromagnetic method was used to detect the water abundance of the goaf in Majiayan Coal Mine, and the detection results were consistent with the actual results.