A derailleur cable detecting assembly for an electric-auxiliary bicycle has a body and a detecting device. The body is mounted on a frame of the electric-auxiliary bicycle, is connected to a derailleur cable and has a casing and a cover. The casing has a chamber and a through hole. The cover is detachably mounted on the casing to mount a cable wire of the derailleur cable in the body. The detecting device is mounted in the body, is connected to the derailleur cable and has a circuit board and a magnetic element. The circuit board is mounted in the chamber of the casing, faces the cable wire and has an electric wire and multiple magnetic sensors. The magnetic sensors are mounted on the circuit board at intervals. The magnetic element is mounted on the cable wire between the casing and the cover, facing the magnetic sensors of the circuit board.