On the basis of the law of energy conservation, we analyzed a diffraction integral formula for the converging light field, derived an inclination factor of the diffraction integral formula for a semi-spherical reference wave front of the diffraction source, and suggested a formula for calculating the light field distribution on the focal plane. In addition, we explained the diffraction process of the converging light field by the interference process of suppositional vector wavelets. Then, according to the wave front transformation properties of focusing objective lens satisfying the sine condition, a calculation formula for the light field in focal plane which focuses from a planar wave front by the focusing objective lens was proposed. Finally, the applications of the above formulas were discussed through the focusing characteristics of the Laguerre-Gaussian beams with simple rectangular coordinate polarization and simple cylindrical coordinate polarization.