A sensing method and system for Fresnel lenses are disclosed, the system including a first Fresnel lens unit for sensing the signal of a target object crossing a first boundary of a sensing area, a second Fresnel lens unit for sensing the signal of a target object crossing a second boundary of the sensing area, a third Fresnel lens unit for sensing the signal of a target object crossing a third boundary of the sensing area, and a sensing unit for receiving and processing the sensed signals of the Fresnel lens units. The first and second Fresnel lens units are arranged on two sides of the third Fresnel lens unit. Furthermore, it is possible to add more Fresnel lens units above, below or around the Fresnel lens units. By monitoring the boundaries of the sensing area, the sensing method and system for the Fresnel lenses improve the insufficiency of the sensing area, reduce cost and save power consumption.