The invention relates to a detection device for an underground horizontal salt rock cavity, which belongs to detection and utilization of underground deep special space. The detection device is characterized in that the detection device is designed according the sonar principle, can move while measuring, can measure the geometrical characteristics of an underground waste horizontal salt rock old cavity in an underground deep waste horizontal salt rock cavity and automatically advances to feed the measuring information immediately to the ground. The detection device is not only important innovation of underground salt rock cavity detection device and method, but also provides new safety control device and method for the control of the disaster of underground water filling space and rescue of life property. The shape and size of waste horizontal salt rock cavity holographically can be measured by using an underwater moving device newly developed to overcome the defects and deficiencies that the existing detecting devices are large in size and cannot measure the horizontal cavity, and the online detection for all water filling spaces that are hundreds of meters deep underground can be performed in time.