An accurate method for measuring the refractive index and thickness of a dielectric film in a leakage waveguide is presented in details. The dispersion equation of a four-layers structure is derived. The complex propagation constant is determined with the Newton-Raphson numerical method. With the dispersion equation derived above , the method for measuring the optical parameters of the single-layer is presented. As an example, the refractive index and thickness of the single-layer silica film deposited with Plasma Enhanced Chemical Vapor Deposition are determined by the present method. The result shows that the present method is accurate and fast compared with the traditional method .To determine the parameters of a sample, a personal computer only spends 60ms using the present method.