A tapping detecting device, a tapping detecting method and a smart projecting system using the same are provided. The tapping detecting device comprises a fingertip size providing unit, a laser emitting unit, a laser sensing unit and a processing unit. The fingertip size providing unit is used for providing the size of a fingertip of a finger. The laser emitting unit is used for providing a laser at a parallel surface above a plane. The laser sensing unit is used for capturing a reflecting image which shows the reflection of the laser caused by the fingertip. The processing unit is used for obtaining a reflecting region according to the reflecting image, and for adjusting a threshold value according to the size of the fingertip. If the area of the reflecting region is larger than the threshold value, then the processing unit deems that the fingertip touches the plane.
