In this paper, a new method of establishing 3-D optical diffraction integrals for focusing optical systems is presented. The method is simple. With these integrals, the vectorial properties of focused diffraction field of the optical systems, especially large aperture optical systems can be easily and effectively studied. Distributions of the light vector and its components on the fooal plane of the optical system are given. Impact of aberrations on the distributions of the light vector and relations between the distributions and the symmetry of the aberration are also studied. Some conclusions are reached.