The axial response of conventional confocal microscope is affected by the tilt angle of the surface to be measured. When the ultraprecisely machined surface is measured with pointing method based on the differential confocal microscope, the tilt angle of the surface to be measured influences the pointing signal, as well as the measuring accuracy and resolution of the treated surface. On the basis of the analysis of the theoretical model of the output signal of differential confocal microscope, it is concluded that the zero point is independent of the tilt angle of surface to be measured and the resolution of the pointing signal is not affected with the tilt angle in a certain range. The experimental results are also presented.