Abstract: Laser frequency splitting technology can be used to measure phase retardation of a wave-plate.With the wave-plate inserted into a laser cavity to split longitudinal laser,its phase retardation can be calculated by detecting the frequency differences between the same order or the adjacent order split longitudinal modes.No mechanical parameters but only the frequencies of optical beats need to be measured,and the measurement can be traced to the wavelength of the laser.Experiments prove that a resolution of 0.003° and a repeatability of 0.05° can be ensured.For a quartz quarter wave-plate of 1.5mm thick,the incertitude of the system is about 1′.