A spatial 3-D nonlinear correction technique is put forward to correct the nonlinearity errors of the PSD in long distance laser collimating measurement. An automatic calibration system is developed to measure the nonlinearity of a 2-D PSD in 3-D space. The nonlinear characteristic of 2-D PSD is checked in a long collimating distance of 78 meters. The experirnental results show that the nonlinearity of 2-D PSD is different evidently when the PSD element is at different position from the laser head. The spatial 3-D mapping relationship between the actual displacements of the incident light and the coordinates of 2-D PSD outputs is established by using a muhilayer feedforward neural network and the nonlinear errors are corrected with this map. The readout errors of the PSD after correction are shown as random and no more than i0. 012 mm in the whole measuring distance.