From the viewpoint of classical electrodynamics. The role of spin-to-orbital angular momentum conservation in spin Hall effect (SHE) of light is identified. The qualitative influence of polarization state, refractive-index gradient, incident angle upon the transverse shift of beam centroid is investigated: If the angle of incidence is the same, the absolute value of the horizontal polarization transverse shift is larger than that of the vertical polarization when the refractive index gradient is positive, but when the refractive index gradient is negative, the situation is exactly opposite. The transverse shift can be modulated by altering the refractive index gradient, the SHE of light can be enhanced obviously with the increase of the incident angle; For a certain circularly polarized component, the transverse shifts of left-handed and right-handed circularly polarized light have same magnitudes but the opposite signs. These findings provide a pathway for modulating the SHE of light.