The accuracy of the phase retardation of a wave plate, which is an important technical parameter, directly affects the performance of the entire polarization optical system. It may be necessary to measure the wave plate before use. Based on the curve distribution characteristics of the polarization interference spectrum, a method for measuring the phase retardation of a wave plate is presented. In this method, the wave plate to be measured is placed between a polarizer and an analyzer, and the transmission spectrum curve in a certain spectral range is measured using a spectrophotometer. Based on an accurate estimation of the wavelengths of certain points with specific transmission from the transmission spectrum curve, the thickness, absolute phase retardation, order, and effective phase retardation of the wave plate can be obtained simultaneously. Theoretical analysis and experimental results show that the proposed method has advantages such as, suitability for zero-order or multi-order crystal wave plate with any phase retardation, high measurement precision, and ease of operation. Furthermore, it has no strict requirements for the directions of the fast axis of the wave plate and the transmission axes of the polarizer and the analyzer.