The Xiaokou rock salt deposit of Guyuan is a large sedimentary deposit found in recent years in Ningxia. In order to detect underground complex geological structure, the authors tried to use the controlled source audio-frequency magnetotelluric sounding method as a exploration means. After feasibility test, the observation device was used as follows: the transceiver distance was 10km, the emitter distance was 1km, and the site distance was 200m. After smooth filtering, static correction and 2D inversion for row data and on the basis of the picture of 2D inversion profile, in combination with the physical parameters and geological data in the survey area, the authors conducted interpretation of the subsurface geological structure. Drilling at the later stage verified the application effect of this method in the survey area.