To realize the fast ignition experiments, deep and quantitative research needs to be taken on the large aperture off-axis paraboloidal mirror (OAP) which is the core component in the high-energy petawatt laser focusing system, in order to provide an accurate basic theory for the OAP structure model selection and precision adjustments. This paper uses a numerical calculation method based on the rigorous vector diffraction theory combining with wavefront aberration analysis, by which the physical optical imaging characteristics of a large aperture and far off-axis OAP are obtained. First, using parallel light incidence, a conclusion can be obtained that the tolerances of OAP translation and revolving around its symmetric axis are closely related with its focal depth. When the optical axis misalignment occurs, astigmatism will play a leading role in the influence on the optical spot which can be described qualitatively by a structural factor. Then, when the incident light has a divergence angle, the best image plane location will change and coma is produced to decrease the power density of the optical spot owing to the asymmetry of OAP. The discussion of the influence affected synchronously by the structural parameters of OAP is also presented.