An on-line space detecting device for wheel inner, which comprises a frame, a servo motor A, a synchronous gear A, a bottom plate, a synchronous belt, a synchronous gear B, a base, a connecting shaft, a servo motor B, a shaft sleeve A, a lower end cap, a connecting shaft B, a shaft sleeve B, an oil cylinder, a bearing A, an end cap, a gland, bearings B, a chassis, a flange, a locating pin, springs, rising flaps, a connecting shaft C, a protection, a rising core etc., the present disclosure can meet the needs of the space detection for wheel inner, and has the characteristics of simple structure, convenient manufacture, stable performance and precision of meeting the processing requirements, and can meet the needs of automatic production.