In modern digital phase-shifting interferometry (PSI), the phase-shifting error of piezoelectric transducer (PZT) optical phase shifter, which is the basic device of phase-shifting interferometer, directly effects measurement accuracy during wavefront phase unwrapping with phase-shifting interferograms. A phenomenon named sub-processional motion that can result in fringes′ rotation in the processing of phase shift is described. This phenomenon is derived from the normal-line rotation of standard reference surface revolving around the elongate direction of PZT phase shifter. In terms of the classic phase-shifting algorithm-Hariharan 5-Bucket algorithm, the error formula of phase unwrapping is given due to sub-processional motion, and the error distribution on aperture is also derived. Based on error factor analysis, criterions to control the error are presented at such aspects