The diffraction field will show aberration if an axicon has astigmatism for beams incident at an angle. In this paper, the diffractional optical field expression of an axicon with astigmatism was derived based on the diffraction integral theory. The influence of the astigmatism on the diffraction property of the axicon was analyzed through the numerical simulation of the diffraction pattern with different astigmatism and the optical intensity distribution along the propagation direction for a given astigmatism. The method of correcting the beam aberration caused by the astigmatism is proposed and realized by the experiment using a precise tunable rotator. Experimental results agree with the theoretical analysis. The result extends the field of applications of the axicon.