A vehicle-mounted detecting device for a bicycle comprises a force sensing base, a strain touch member for detecting the direction and magnitude of force, and a sensor contacted with the sensor. The detecting device is assembled to a crank and a pedal shaft of the bicycle in a manner of external hanging. The detecting device may obtain a pedaling force detecting message. The message of the rotating speed and the tilted angle message may be obtained by a cadence sensor which is packaged in a ring housing. The messages are finally transmitted to a smart mobile device by a microprocessor which is mounted at a ring outer base.