In practical measurement, the positioning precision of position sensitive detector (PSD) is greatly affected by the size and asymmetry of the spot. A three-ray positioning model of PSD caused by laser spot is established systematically based on the Gauss energy distribution character of laser beam. The positioning error is derived under the character of the spot by its shape. The validity of the PSD positioning model is proved by experiments. The positioning errors caused by the asymmetry of the spot, the change of the spot size in transmission, and the speckle caused by the interaction between the laser and object can be compensated by using this positioning model in laser triangulation measurement. And the precision of positioning can be improved effectively. Experiments show that the compensated error can reach 0.07 mm.