By focusing a laser beam through a high-numerical aperture objective, a sub-wavelength focal spot is obtained, which enables the application of a laser beam in microscopy, lithography and optical data storage. In this paper, the focusing properties of a laser beam through a high-numerical aperture are studied based on the vectorial Debye diffraction theory. The intensity, phase, degree of coherence and degree of polarization in the focal field are analyzed systematically. The influence of the polarization and phase state of the incident beam on the shape of the focal spot is also investigated. It is found that the spin angular momentum of the incident laser beam will convert to the orbital angular momentum while focusing. Meanwhile, the degree of coherence and the degree of polarization of a beam can change during the focusing process. Moreover, by modulating the polarization and phase state of the incident beam, the desired shape of focusing spots can be obtained.