Abstract: The influence of resonant cavity of a resonance detector on the sensitivity of a surface acoustic wave (SAW) gas chromatograph (GC) gas sensor system was investigated. The 3-dimentional finite element method (FEM) models of the absence and presence of a liquid layer covering the electrode structures were established. By using the FEM the coupling-of-modes parameters in different periodic structures were extracted. The variation of sensors' sensitivity with different cavity length was deduced by using P-matrix cascading technique. In the experiments, two three-transducer- structure SAW resonators were fabricated, where the distances between input transducer and output transducer were 22.25 times the wavelength and 1.25 times the wavelength, respectively By detecting the same amount of dimethyl methylphosphonate the theoretical results were verified. The results showed that shortening the resonant cavity of a SAW resonator concentrated more acoustic energy towards the center area, realizing higher sensitivity of the SAW/GC gas sensor system.