The pinciple of position sensitive detector (PSD) is briefly introduced and some experiments for studing the effects of light source on the position precision of PSD are performed as wel1. It is proved that the fluctuation of light intensity can lead to the loss of the position precision of PSD, the results are far from those derived from theoretical analysis. At the same time, the experiments show that the backgroung niose can also exert certain effects on the ultimate position precision.