By the method of rotating quarter-wave plate,an automatic measurement system for the Stokes parameters of polarization state has been set up. It can process and display 15 polarization ellipses per second. The virtual instrument in this system consists of a data acquisition card and the LabVIEW software. The rotating angle is determined precisely by data interpolation and resample,so that the systematic error caused by the instability of rotating motor is depressed. The variation equation of transmitting light intensity with the rotating angle is deduced theoretically by Mueller matrix. The real-time acquired data is compared and fitted with the analytic equation. The coefficients of the variation equation and the correspondent Stokes parameters are given by Fourier transform. The field components and their phase difference of light wave are calculated. The distinction ratio,long axis orientation and spin-wise of polarization ellipse are determined. The influence of background noise on the measurement result is examined. The performances are compared between the two methods of rotating wave-plate and rotating analyzer.