Abstract: This article introduces a threeinone composite achromatic compensator.The continuous adjustment of retardation from 0 to 2π can be realized by rotating one of the wave plates.As for the compensator,it is characteristic of convenient adjustment, big adjusting range and high degree of accuracy. The theory and experiment show that the most important feature of this composite compensator is its achromaticity in a certain spectral range. the compensator can be used to polychromatic light because its retardation is independent of wavelength.