The focusing properties of partially coherent and circularly polarized Bessel-Gaussian beams through a high numerical-aperture objective are studied. Based on vectorial Debye theory, expressions of intensity distribution and degree of polarization in the focal field are derived. Numerical calculations are performed to compare the different focusing properties of right-circular and left-circular polarized partially coherent Bessel-Gaussian beams. Furthermore, the influence of correlated parameters of the incident beam and the maximal angle of the high numerical-aperture objective are studied in great detail. It is found that the spin angular momentum of the beam will transfer into orbital angular momentum when the circularly polarized beam is focused by a high numerical-aperture objective. It is also found that the focusing properties will be influenced by the correlated parameters studied. The research of this paper is of great importance in the optical micromanipulation field.