Abstract: With the application of sonar seepage detection technology in hydrogeological exploration, the accuracy of the measurement results plays an important role in obtaining accurate hydrologic parameters. Based on a foundation pit project in Nanning of Guangxi, this paper adopts the method of field test and considers the combination of natural flow field and artificial flow field. Sonar seepage detection was carried out on the foundation pit waterproof curtain in three stages, and the exposure of leakage defects at different stages is compared. The accuracy of the measured results is judged according to the change of the magnitude of osmotic velocity. The correctness is verified in the actual engineering progress. The research results show that the permeability velocity varied significantly in different precipitation stages, and with each 10 m increase in the depth of foundation pit water level, the seepage velocity detected by sonar seepage increases by an average of 1 to 2 orders of magnitude. The reason is that the difference between the water head inside and outside the foundation pit is increased due to precipitation, and the increase of hydraulic gradient makes the seepage field near the leakage defect change. Under the action of high water head, the seepage velocity changes obviously, so the location of the leakage defect is more accurate, and the accuracy of sonar detection is also higher. Therefore, it is very necessary to raise the head difference between the inside and outside of the foundation pit to accurately locate the wall leakage defects. When the sonar seepage detection is adopted, it is recommended to lower the groundwater level below the foundation pit floor, so as to effectively detect the risk of seepage of waterproof curtain.