When the measured surface is sloping,the coaxial condition is no longer met.Here,the response of confocal system derived from Fresnel is not complete.The theoretical model of confocal microscopy axial property is first inferred from the Kirchoff diffraction formula when the normal of the measured surface is not parallel with the optical axis of a confocal system.The theoretic analyses show that the full width at half maximums(FWHM) of different slopes are the same,that is the FWHM of confocal axial response is irrelevant to tilt angle.The model is simulated numerically,and the result accords with the experiments.