Based on the theory of polarization interference, a method for measuring phase retardation and thickness of a wave plate in a wide spectral range is proposed. The relationships of the intersection wavelengths of spectral transmission curve and median transmission line are analyzed by matrix optics method, and the formulas for calculating the phase retardation and thickness of the wave plate are given. Error is also analyzed and it is shown that the maximum error of the measurement of phase retardation and thickness are 3.38°and 0.66μm respectively. The experiment results measured by a spectrophotometer show the validity of this method. The method can measure multiple physical parameters of the wave plate simultaneously, and has some advantages, such as, has no strict requirement for the directions of transmission axes of the polarizer and the analyzer and the fast axis of the wave plate, has no damage and pollution to the wave plate. The method has some application value in the fields of wave plate fabrication and quality evaluation before using a wave plate.