Based on vectorial Debye theory, the expressions for the distribution of the light field and orbital angular momentum (OAM) of tightly focused vortex beams have been derived. A Bessel-Gaussian (BG) beam is set as the model of the vortex beam. The phase distribution of the field component in the x direction near focus is analyzed as usual by presenting phase contours. It is found that circular edge dislocations exist in the focal plane, but they evolve as screw wavefront dislocations in observation planes away from the focus. Moreover, the dependence of the distribution of phase contours and OAM in the focal plane on the numerical aperture is shown. The distribution of OAM along the z axis is studied; it is found that the optimal location for applications of OAM of vortex beams along the z axis is near the focal plane.