When the axicon is tilted, the incident plane wave focused by it does not form a Bessel distribution diffraction pattern again. The Kirchholf diffraction formula is needed to analyze the diffraction result. Some higher term concerning with the tilted angle in the expansion of phase exponent should be retained. After conducting, an integral about φ can be obtained. The integral result fitted with the real diffraction pattern very well. Numeral simulation and experimental result agree with each other. The diffraction pattern formula can be used to estimate the adjustment permissible error of axicon in application.