A novel method used for the enhancement of the accuracy of the measurement of the retardance of a quarter-wave plate employing two polaroids and a prism is reported under the condition of knowing the direction of the fast axis of the plate. The theoretical analysis of the principle and the uncertainty formula are given. An application example is also demonstrated. The measured result of the example is verified with an experiment. The main advantages of this method include the accuracy enhancement, simple measuring setup and easy operation.