Before the implementation of engineering projects, it is essential to determine the detailed distribution of underground spaces in the work area. The deployment of a 3D high-density electrical observation system is difficult, and using conventional electrical equipment for acquisition is time-consuming, especially in complex geological conditions. By using parallel electrical acquisition technology, efficient data acquisition can be achieved. To accurately determine the three-dimensional morphology of the area where possible leakage pipes may form in the lower part and surrounding areas of a phosphogypsum stacking yard, parallel electrical acquisition was used to collect data, combined with actual geological conditions. The three-dimensional electrical exploration data volume under the true three-dimensional observation system in the measurement area was obtained, and the data was processed and interpreted with three-dimensional inversion with terrain. The results show that 3D parallel high-density electrical observation can effectively and accurately reveal the distribution of underground spaces on the site. A total of 5 abnormal areas were delineated during the on-site exploration. The results were consistent with the drilling exposure data and provided accurate basis for guiding the construction of anti-seepage curtains in the later stage.