The positions of the impurity K ion in the crystal are simulated by computer technology. The various kinds of defects corresponding in the K:PbWO 4 are calculated. The defect chemistry and the defect reactions with the different impurity concentration doping PWO crystal have been studied. The origin of the decrease of the 350nm absorption band of K:PbWO 4 under low concentration is discussed, and the previous experimental results recur by computer simulation. The calculated results show that the 420nm absorption band will be eliminated and the property of the light-induced refractive will be enhanced under the condition of heavily doped K:PbWO 4 .