A theoretical method of getting COM parameters is presented. The admittance curve of a one port synchronous SAW resonator is calculated using finite FEM/BEM, according to the characteristics of SAW dispersion, which is used to extract all the COM parameters through four steps:Algebraic analysis;Univariate function optimization;Univariate function optimization once again;Multivariate function optimization. Comparing with traditional methods, this approach is convenient, stable and reliable. The simulation of a hybrid SAW filter using the obtained COM parameters is consistent with the result directly calculated by FEM/BEM, which verifies the accuracy of COM parameters by our method.