The invention discloses a detecting device for side walls of tunnels. The detecting device comprises a geological radar device, an expansion mechanism and a base, the geological radar device is mounted on the base by the expansion mechanism, the expansion mechanism consists of a guide rail, a guide rail slider, a brace and a driving assembly, the guide rail is obliquely arranged on the base, the brace is slidably mounted on the guide rail via the guide rail slider, the driving assembly drives the brace to slide on the guide rail, and the geological radar device is mounted on the top of the brace by a buffering mechanism and can elastically contact with a side wall of a tunnel via the buffering mechanism. By the aid of the additional expansion mechanism, the geological radar device does not need to be held by a worker to detect the side wall, and detection safety and stability are improved. The detecting device is applicable to detecting the tunnels.