A guide ring sensor has a fixing seat, a guide ring, an elastic member, and a sensing element. The guide ring is pivoted on the fixing seat by a positioning member. The elastic member is mounted on the body of the fixing seat, is connected to the guide ring, and provides resilience to the guide ring. The sensing element is mounted on the fixing seat to detect a pivoting angle change of the guide ring and generate a signal corresponding to the pivoting angle change of the guide ring. The sensing element may detect whether a webbing is correctly fastened or not by the pivoting angle change of the guide ring.