In the canse of engineering slope with confocal microscopy, the paraxial condition is not suitable. The conclusion derived from Fresnel approximation condition is not suitable for this situation. The axial response of confocal microscopy of slope measurement is first acquired with the application of Kirchhoff diffraction formula, and the axial response of confocal microscopy derived from Fresnel approximation condition is just a special case of the general module given in this paper. Meanwhile, as the detectors are arranged in a differential way, the theoretical module of measuring signal is obtained and analyzed. Some slopes with different angles are measured and a series of experimental curves are presented. With numerical calculation, the conclusion that experimental result accords with the theoretical analysis is inferred. The slope of 10° angle block gauge is profiled with resolution of 20 nanometer with the differential confocal microscopic system.