Abstract: Effects of certain impurities, which exit in growth solution of KDP crystal, on optical qualities of KDP crystal were investigated. The results showed that the effects of different kinds of impurities are not the same. The reason can be ascribed to the chemical structure and chemical bonds between the impurities and surface atoms of KDP crystal are not same．Those impurities have similar structure and easy to form H bonds with ［H２PO４］-,such as pyrophosphate and sulfate affect pyramidal sector of KDP crystal easily while the other impurities, such as Cl- have no evident effect on the growth and properties of KDP crystals.