Reflective off-axis parabolic mirror has been widely used in radiation calibration, broadband target simulation and energy focusing, because it does not produce chromatic-aberration and has bigger available aperture. The basic factors affecting collimating characteristic are off-axis magnitude, effective aperture and emission position on the focal plane of off-axis parabolic mirror. Vector expression is adopted in analyzing the relation between off-axis parabolic parameters and collimation property. Curves for collimation property of the off-axis collimator are given out in three aspects: the emission position on focal plane varied from 0 to 0.05f, the relative aperture D/f varied from 0 to 0.40 and the relative off-axis magnitude h/f varied from 0.1 to 0.5. Analysis results obtained through vector method are validated by CODE V and tally with those from CODE V under the same conditions. The results are helpful to design parabolic collimators.