Based on the Richards-Wolf vector diffraction integral theory and the conclusion that an angularly polarized vortex beam has a radial component after strong focusing with a high numerical aperture, the 4pi focusing characteristics of an angularly polarized Laguerre-Gaussian beam modulated by a diffraction optical element (DOE) are studied. The simulation results show that the flat-top optical field can be obtained in the focal plane by selecting the appropriate intercept ratio, numerical aperture, and outer ring structure of DOE. On this basis, by changing any one of these three parameters, the structure of the flat-topped optical field will be transformed into an optical needle one. By changing any two of these three parameters, a good dark channel structure can be obtained, and all the dark channel structures obtained are basically same. The conversion among these special focal field structures has a potential application in the field of optical micro-manipulation.