Abstract： Utilizing the differences of the knocking sound between the hollowing wall and normal wall, a detecting system was designed with a wall clamping robot as its carrier. After filtering the sound signals with HPF (high pass filter) in matlab programing, the signals were processed and the characteristic parameters of the signals were obtained. After comparing the results with the parameter of the normal signal, the decision was made whether the hollowing phenomenon exists. The experiment results show that this method is easy to use with high precision, and it can replace human beings to complete dangerous tasks.