On the basis of building the mathematic light-path model of four-quadrant photodetector, three dimensional(3D) position monitoring system of an object is built. For the 3D position, pitching, orientation and rolling departure are included. The monitoring results indicate that position monitoring with four-quadrant photodetector is practical, economical and precise. And the testing precision can be corrected to the degree of second. So the theoretic basis for the further research and application of photodetector without calibrating experiment sample data is founded.