The changes of defects in PbWO4 crystal caused by La dopant have been studied by means of positron annihilation lifetime and X-ray photoelectron spec trum (XPS).The results show that La dopant enhance the concentration of lead vac ancy (ＶPb) which can be described as the positron capture center in PbWO4 crystal,and lead vacancy will furthermore introduce low-valent oxygen center.We discuss the mechanism of La doped in PbWO4,and consi der that oxygen vacancy is restrained by doping of La,while lead vacancy density is increased by La dopant.