In order to improve the position precision, a single analytical expression for position errors was presented in terms of the various error contributors, including phase measuring interferometer errors, wavefront errors of transmission sphere, surface deviation and adjustment errors. The contrastive experiments indicate that after controlling the adjustment errors, the linear trend of power and radius is changed to the random trend, and the radius variation is decreased from 7.05 μm to 0.5 μm. The adjustment errors are the main error contributors. The power can be effectively controled by minishing adjustment errors. Thus the interferometric radius precision is improved.