High order Bessel beam can be generated by using spiral phase plate-axicon system. Based on Kirchhoff diffraction integral theory, the expression of diffraction light field after the axicon illuminated by an oblique vortex beam is derived. The influence of astigmatism introduced by oblique illumination of light on high order Bessel beam generated by axicon fucusing vortex beams is analyzed. A simple and feasible scheme for detecting topological charge number is proposed. Results show that when the deflection angle of axicon is small, the center bright ring of high order Bessel beam becomes elliptical. The ellipticity of center bright ring increases with the increasing of reflection angle, and the phenomenon of dark core splitting can be found. With the continuous increasing of the reflection angle, the bright ring will split one by one from inside to outside, and ultimately it will evolve into caustics beam with point array structure. Experiments are designed to verify above researches. Experimental results are in good agreement with theoretical analysis and numerical simulation.