A two-dimensional, position-sensitive sensor-based system for positioning an object having six degrees of freedom in space, used for positioning of a silicon table and mask table of a lithography machine. The system comprises mainly a semiconductor laser 1, an optical fiber collimator 2, optical fibers 3, 7, 10, and 13, an optical fiber splitter 4, filter plates 6, 9, and 12, three PSD sensors 5, 8, and 11, and a signal processing system. Laser emitted by the semiconductor laser 1 is irradiated onto the optical fiber collimator 2, then transmitted respectively via three paths, and received by the PSD sensors 5, 8, and 11 after having background light filtered out by the filter plates 6, 9, and 12, while the positions of laser spots on the three PSD sensors 5, 8, and 11 are processed by the signal processing system to acquire the position and orientation.