Abstract: To measure absolute length, the synthetic wavelength methods must construct a long enough synthetic wavelength made use of several single wavelengths, and make integer of the interference order be zero or a deterministic value. However, it is not easy to find corresponding wavelengths with appropriate values and high wavelength stability in practice. In this paper, a synthetic wavelength absolute measurement method was presented, adopting three high frequency stability lasers to form the synthetic wavelength chain. The measurement equations as well as the transition conditions between grades were derived. And a corresponding phase shift interference optical path was built to make high precision measurement for the absolute length value of the sample. The experiment and error analysis results show the measurement uncertainty is about 3 nm (at 0.3 m sample). In this method the requirement for initial measurement accuracy of the length is low and the measurement range is only limited by the length of the interferometric optical path.