Four-zone method to measure the phase retardation of wave plate is put forward based on the polarizer-wave plate-analyzer system. Four sets of light intensity were collected by adjusting azimuth angles of measured wave plate and analyzer. The phase retardation of the wave plate is figured out with linear calculation. This method eliminates the error caused by polarizer and analyzer imperfections completely in phase retardation measurement. No standard wave plate or other phase modulation instruments are necessary in the system. The measured wavelength range is extended, because it is only related to polarizing prisms and detectors. With this measuring system, the instrument error σ is less than±3.49065×10-3 rad (about 0.2°). The accuracy is increased by about one order of magnitude compared with other algorithms which is verified by experiments.