The Kretschmann module comprises a prism and metal film, and a substrate module comprises two metal films and a glass sheet. When the two modules are close to each other, a double-sided metal-cladding waveguide structure with a nano air gap and an organic film as the waveguide layer will be formed. The resonance angle of the guide mode resonance excited in the double-sided metal-cladding waveguide structure is linearly related to the thickness of the nano air gap. When the reflectivity curve of one of the modules is selected as the detection signal, the thickness of the nano air gap can be measured by measuring the change in the resonance angle. Numerical simulation calculation results show that this structure can measure the air gap in the range of 0--100 nm, and the measurement resolution can reach 1 nm.