The retardation error is the main error of Stokes parameters measurement based on rotating quarter-wave method. The principle of Stokes parameters measurement based on conventional rotating quarter-wave method is analyzed theoretically. The relation between the retardation error and the measurement error is discussed. An improved rotating quarter-wave method is proposed for improving the measurement accuracy of Stokes parameters. In this method, the angle between the axis of polarizer and the polarization direction of the light is adjusted to a certain value (90° or 45°), and the influence of the retardation error on the measurement accuracy of Stokes parameters is reduced obviously. The effectiveness of the proposed method is investigated by comparing to the conventional rotating quarter-wave method through experiments. Experimental results show that the measurement error of Stokes parameters is reduced by the proposed method from 4.31% to 0.33%, under the condition that the retardation error is lower than 1.2°.