Aiming at the application of precision length measurement, the error of phase-shifting algorithm based on the cosine dependent algoritm by means of double-beam interference is explored, and the precision formula of intensity distribution of multiple-beam interference (multiple reflection and transmission by the optical surface) with the basic principle of interference optics is derived. Based on this formula and the numerical analysis, the maximum error of the interference intensity is found to reach 14.4% due to ignoring the second multiple-beam interference in the precision measurement, and the different dependences between the reflection coefficients of the two surface and the phase difference for the four-step algorithm and the five-step algorithm are listed: the latter possesses higher accuracy than the former since the former is more sensitive to the intensity error; and the error of the algorithm can be ignored while the reflection coefficient of the two surfaces or one of them is low, unless the reflectivity of the two surfaces is high.