By use of vectorial diffraction theory, intensity distribution around focal spot of stimulated emission depletion (STED) beam in STED microscopy is numerically calculated and influence of diverse 0/π circular phase plate to performance of focal spot is evaluated. It is demonstrated that dark spot in center of focal plane can never be expected unless proper parameter of phase plate is selected, while such parameters are diverse with various polarizations and beam shapes of incident beams. Furthermore, effect of numerical aperture is comparatively neglectable. Explicit calculation further presents that reasonable value ranges of the inner radii of the phase plate are almost the same if the STED beam is linearly or circularly polarized, which is smaller than that of system with a radially polarized incidenct beam and the beam with azimuthal polarization can never be the proper option for STED beam.