Stokes ellipsometer can measure the optical parameters of thin films rapidly. The precision of the instrument matrix of Stokes ellipsometer directly affects the measurement of the Stokes parameters of the light reflected by films and indirectly limits the measurement accuracy of the optical parameters of films. It is studied that how to generate elliptically polarized light with a combination of linearly polarized light and standard films theoretically. According to the four-point calibration method, the instrument matrix with combination of linearly polarized light and standard films in place are calculated, and the errors caused by the azimuth of optical components and its defects in traditional calibration methods are avoided effectively, thereby improving measurement accuracy of the optical parameters of thin films. The experimental results show that, in the method, the deviation of the measurement of Stokes parameters of light reflected by the film is less than 0.6% and the measurement deviations of thickness and refractive index are less than 0.2 nm, 0.003 repectively.