With differential confocal pointing method, the ultra-precise machined surface can be measured with high resolution. In this technology, the differential confocal pointing signal, the zero point of the output diagram, may be affected by the different response of the two detectors, the different defocus of the two detectors, the accordance of the two pinholes in front of the detectors and so on. The paper aims at discussing the effect of these elements on the measuring signal. Through theoretical analysis of the output of the differential confocal microscopy (DCM), the conclusion that the pointing signal keeps fixed after the differential confocal system has been set up, does not change with the differences mentioned above, is presented. However, the resolution of the differential confocal system is concerned with the response performance and defocus of the two detectors. This is also confirmed by experiments.