To tune the operating frequency of a Frequency Selective Surface (FSS), the load impedance of the loop slot FSS forms was separated to an inductive surface and a capacitive surface, and the coupling mechanism between the two surfaces was used to develop a Complementary FSS(CFSS). An equivalent circuit model of the CFSS was established and its tunable resonance mechanism was analyzed quantitatively. The influences of the rotating angle of the load patch, coupled dielectric thickness and the relative dielectric constant on the transmission characteristics of the CFSS were calculated accurately by using the coupled integral equation method, then the inductive surface and the capacitive surface were fabricated on both sides of the coupled dielectric by the coating and lithography methods. The transmission response of a 250 mm × 250 mm CFSS prototype was measured by using the free space method. The calculated and measured results show that the CFSS resonance will be tuned from 18.2 GHz to 14.8 GHz by rotating the cross patch from 0 degree to 10 degree. The coupling effect between the capacitive surface and the inductive surface will disappear by increasing the physical thickness of the coupling dielectric from 0.1 mm to 1 mm. With increasing the relative dielectric constant of the coupling medium, the coupling between the CFSSs will be strengthened and the operating frequency will shift to lower frequencies. It concludes that the CFSS can realize the active resonance conversion function as the rotating angle changes, which provides references for the design and manufacture of active FSSs.