A radiator detecting device of the present invention includes a radiator cap, a sensing unit, a circuit device, a warning device, and an adjusting means. One end of the sensing unit connects to the radiator cap, and another end extends toward a direction away from the radiator cap. The circuit device has a circuit board. The circuit board includes a control circuit for detecting a state of the sensing unit, and the sensing unit selectively makes the control circuit generate a suggestion signal. Once receiving the suggestion signal generated by the control circuit, the warning device generates a warning signal. The adjusting means is used for adjusting a length of the sensing unit received in the radiator. Said sensing unit can be moved with respect to the radiator between a first position and a second position. Thereby, the sensing unit can fit different depths of different radiators.