Abstract: It is particularly important for the underground engineering design and construction in karst development areas to identify the refined structure of hollow areas and underground morphology based on the structural characteristics of underground rock mass and survey techniques in deep voids. In order to further understand the “black box” structure of deep underground rock mass and master key technology of accurate exploration in underground space, this article introduces a detection system by borehole ultrasonic synthetic aperture imaging and multi-frequency scanning. According to the synergistic effect of optics and acoustics inside the borehole, this system derived a multi-frequency ultrasonic scanning technology suitable for detecting complex geological borehole rock mass, and designed a comprehensive survey equipment for borehole rock mass structure with intelligent perception and recognition capabilities. In addition, the system can synchronously survey borehole rock walls, caves inside the borehole, and near-field surrounding rocks in complex geological environments. Combined with its supporting analysis methods, we have constructed a comprehensive survey system for borehole rock mass structure integrating “observation, measurement, and characterization”. The test results show that the ultrasonic synthetic aperture imaging and multi-frequency scanning detection inside the borehole can perform 360° precise detection by going into inaccessible underground spaces along the borehole, enriching the survey data of rock structure in underground space development and construction, which has forward-looking and wide application prospects.