The shape and size characteristics of underground cavity are of great significance to the stability calculation and evaluation of geological structure in the region. However

the borehole sonar currently used lacks universality and it is difficult to meet the requirements of both detection accuracy and detection range. Therefore

on the basis of traditional borehole sonar method

this paper proposes a new method of detecting cavity based on multi-frequency borehole sonar. With considerations given to the contribution of different frequencies to the actual detection object

this method synthesizes the detection data of various frequencies and effectively solves the problem of the opposition between detection scale and detection accuracy. Firstly

the optimal frequency expression of the detection system is derived to provide basic parameters to reflect the influence of different frequencies on the analysis of detection results to a greater extent

and the frequency influence factor parameter is established to describe the deviation degree between the natural frequency and the optimal frequency. Then

on the basis of superposition of the frequency influence factor

the detection data of various detection frequencies are synthesised

and detection objects are accurately measured by using the improved spectrum ranging method. Finally

after adding depth and azimuth information

the three-dimensional reconstruction of cavity shape is realized. The feasibility and accuracy of the method are verified by applying it to practical engineering projects.