In order to test the nanometer films fast, a method of ellipsometry based on Stokes parameters with a Division-of-amplitude Polarimeter (DOAP) was introduced. The structure of instrument, theoretical calculation and parameters testing were investigated. Firstly, a DOAP was designed, in which the light path was in the same plane. This was calibrated by known polarization that combined a polarizer with one quarter-wave plate. Then, the Stokes parameters could be tested quickly by use of four photoelectric detectors with the same properties. Secondly, the ellipsometric parameters (ψ, Δ) of a nanometer film were worked out by testing the Stokes parameters of incidence and reflection light on a sample with the DOAP. Then, the thickness and refractive index of the sample were obtained. The testing error of Stokes parameters was discussed. Finally, the parallel experiment with an ellipsometer in extinction was done. The experiment shows that the related coefficient of the thickness and refractive index are greater than 85% and consistency of two instruments is good. With the conclusion, the nanometer films can be measured fast by testing the polarized states of incidence and reflection light, and the DOAP can be set up easily, adjusted conveniently and calibrated simply.