An absolute distance measurement system based on the selfmixing interferometry of a threewavelength optical fiber laser has been presented and experimented. The optical fiber laser is consisted of three independent laser cavities and there is erbiumdoped fiber as a gain medium in each laser cavity. The three laser cavities use fiber Bragg gratings as the reflectors and wavelength selectors, and three wavelengths have been emitted simultaneously. There is no laser mode competition between the three wavelengths so each of the three wavelengths has stable frequency and power. Absolute distance measurement can be realized based on the selfmixing interferometry of the three wavelengths and the coincidence of the phase decimal of the three wavelengths selfmixing interferometric signals. By the method of simulation analyzing, the difference between every two adjacent wavelengths should be equal approximately. In the experiments, the difference between every two adjacent wavelengths is about 10 nm. A step with the nominal height of 11 mm and the corrected value less than 2.7 μm was measured by the system. The measurement result is 11.000 059 mm. The standard deviation of twenty times repeated measurements of the absolute distance 13.000 090 mm is 4.4 nm.