Aiming at the characteristics of a single crystal silicon sphere diameter measuring system, multiple-beam interference and dual-beam interference of the reflected light of a plane wave and those of the central reflected light of a Gaussian beam with normal incidence are analyzed. Also numerical simulations for the reflected interferential intensity of a Gaussian beam center in different situations are presented. Furthermore, the influence of five-step phase-shifting algorithm on Gaussian multiple-beam interference is studied, and the maximal phase errors are shown with some specific parameters. The maximal phase error is 0.08% for beam waist ω0=5 mm and propagation distance z=2000 mm.