Usual optical phase retardance measurement methods use a single wavelength of light for measurement, which cannot accurately reflect the variation of retardance amount with wavelength. A simple method for measuring phase retardance spectrum is proposed. The method uses plane polarimeter to measure the transmission spectrum of the system by rotating the component to be measured, and then solves the phase retardance spectrum according to the principle algorithm. The experimental results show that this method has good repeatability and simple operation, and the repeatability of the measurement results can be kept within 0.41°.