The positions of the impurity K ion in the crystal are simulated by computer technology.The corresponding various kinds of defects in the K:PbWO_4 are calculated.The defect chemistry and the defect reactions with the different impurity concentration doping PWO crystal are studied.The origin of the decrease of the 350 nm absorption band of K:PbWO_4 under low concentration is discussed,and the previous experimental results are recurred by computer simulation.The calculated results show that the 420 nm absorption band will be eliminated and the property of the light induced refraction will be enhanced under the condition of heavily doped K:PbWO_4.