The frequency response characteristics and the sensitive mechanism of the ST cut quartz based SAW sensor with two-port resonator and FPOL sensitive film were theoretically analyzed.The factors which affect the frequency response and the detective sensitivity were systematically discussed by P-matrix method and the perturbation theory.Conclusion can be drawn that it is necessary to properly choose the width

the thickness and the material characteristics of the sensitive film in order to improve the detective sensitivity and to decrease the insert loss.The optimized device struture and sensitive film parameters were given

which can provide important reference in design and characteristics analyzsis of the SAW gas sensor using two-port resonator.