Variation of solution pH value influences the quality of and the growth patterns of KDP crystals. The solubility curves of KDP (KH2PO4) solutions with differing pH values were obtained by the weighing method. The supersaturation curves of these solutions were observed by surveying the first visible crystals temperature. The widths of the relevant metastable zone were obtained. The results show that KDP solubility increases with the increasing or decreasing growth solution pH value. With these changes, the metastable zone of solutions expands and the stability improves. A slightly higher pH value of KDP solution than normal was found to be more suitable for growing highquality and largesized crystals.