The phase errors, resulting from the multi-beam interference in the Fabry-Perot cavities of the wavelength scanning fiber-optic interferometer for absolute distance measurement, are analyzed. It′s proved that selecting the suitable reflectivities of two F-P cavities and the length of the reference cavity can reduce the phase errors. In authors′ system, the optimal cavity reflectivity is in the range of 0.10～0.15 and the reference cavity length is about 0.95 mm, the accuracy of 0.05 μm can be obtained in the range of 1 mm.