Based on the principle of Feng et al., an improved optical method for measuring small rotational angle in chiral medium is proposed. When a quarter-wave plate and two analyzers with proper azimuth angles are arranged in the two outputs of a Mach-Zehnder interferometer (with sample been inserted in one of the light passages), the final phase difference of the two interference signals used to determine the rotational angle is greatly enhanced, which is found to be a about 15× the result obtained with the Feng et al.'s method. The feasibility of the measuring method was demonstrated by our experimental results. This method should bear the merits of high accuracy, short sample medium length, and simpler operational endeavor.