The invention discloses a detecting device for rifling angle of a small-caliber gun barrel. The base of the device is used as the mounting carrier of the whole device. An adaptor seat is connected with the base through a screw thread. A front sliding copper sleeve and a back sliding copper sleeve respectively sleeve the base and the adaptor seat. A connecting shaft is connected with the other end of the adaptor seat through a screw thread. The other end of the connecting shaft is connected with a locking nut through a screw thread. A coder mounting sheet is fixedly connected with the inner hole of the base through a screw. A coder is snapped on the coder mounting sheet. A bearing seat is fixedly connected with the other end of the base through a screw thread. The detecting device is suitable for detecting the rifling angles of the small-caliber gun barrels with different specifications. Furthermore the detecting device has advantages of simple structure, easy operation, high detecting precision and high suitability for popularization.