A new method for retardation measurement of a quarter-wave plate was proposed. It was realized by calculating the long and short axes' values of the output elliptical polarized light based on computer simulations. The optical measuring system was simply composed of a laser, a polarizer, the quarter-wave plate to be measured, an analyzer and an optical detector. The real retardation can be obtained by a data processing with Mathcad. The principle of this new method was theoretically analyzed and an application example and its error analysis were demonstrated in this paper. The experiment results show that the absolute error is less than 0.26°. Therefore the main advantages of the method are a few optical devices required, very easy to run and a big improvement in measuring accuracy.