Abstract: For detecting elliptically polarized light and partially polarized light,the detection method is analyzed frow the view of the composite retardation plate theory.The results indicate any elliptically polarized light can be compensated to become lineally polarized light throught λ/4 waveplate as long as the fast-axis(slow-axis) of λ/4 wave-plate and the long-axis(short-axis) of elliptically polarized light are in the same orientation,but partially polarized light cann't.Experiments are performanced and the test results accord with theoretical analyses.