Aiming at the application of precision measurement of length, the phase-shift a lgorithm including algorithm error and optimum range of the phase-shift step wa s studied by using the multiple-beam interference principle in this paper. We have derived an accurate formula for the intensity distribution of multiple-be am interference (multiple reflection by and transmission through the optical s urface) in the basic principle of interference optics. Based on this formula and the numerical analysis technique, we have found the optimum value of the phase-shift step and the applicable range of the initial phase difference between in terference beams by using the precision measurement of length with Fizeau inter ferometer.We also found the respective relationships between the original phase error and the phase of four-step algorithm (and of the five-step algorithm) d ue to the intensity error from cosine dependence. We have assessed the uncertain ty from several main error sources of the multiple-beam interference.