A controlling method for a sensing system includes the following step. Whether a specific function is started is determined according to the amount of pixels of a total pattern acquired by sensing a first object. In an embodiment of the present invention, the step of determining whether the specific function is started according to the amount of pixels of the total pattern is the step of determining whether the specific function is started according to the amount of pixels of the total pattern and the length of time to successively acquire the total pattern. Accordingly, it is more convenient for a user to use the sensing system having the said controlling method.