The effect of diffractive optical elements (DOE) modulating the incident light field on the focusing field is studied. The 4π focusing properties of Bessel-Gaussian beams radially polarized after modulated by several different DOEs are analyzed via the Richards-Wolf vector diffraction theory. The numerical simulation results show that multiple spherical spots of potential applications form near the focal plane, and the number of spherical spots is related to that of the belts on DOE, while the DOE transmission function has little influence upon it. Moreover, the number of spherical spots increases with that of the belts on the DOE.