Phosphorus (P) is one of the key nutrients in surface water, and plays a vital role in the material cycle and energy flow in aquatic ecosystems. Hence, it is of great significance to explore the source, transformation, and fate in water body for understanding its evolution process and scientific protection of water environment. Recently, phosphate oxygen isotope technology (δ¹⁸O_P) has been gradually applied in tracing the P source and studying the biogeochemical cycle in freshwater environment, which most used the sample pretreatment of seawater methods. By contrast, the concentrations of PO₄^3- in freshwater samples are usually low, whereas the contents of organic matter and interference ions are high. The pretreatment complexity restricted the extensive application of δ¹⁸O_P analysis in freshwater ecosystems. This study examined the applicability of the existing pretreatment methods of seawater sample in surface freshwater environment, and further carried out three optimization improvements: ①We replaced MgCl₂ solution with Mg(NO₃)₂ solution in the MAGIC precipitation step to avoid the interference of Cl^- and the impurities of AgCl. ②The solution pH value of Ag₃PO₄ precipitation was adjusted to 8.0 to ensure the rapid and complete precipitation. ③Possible photolysis effects of AgNO₃ and Ag₃PO₄ were inhibited by using light avoidance. This modification improved the purity of Ag₃PO₄, and attained more accurate δ¹⁸O_P results. The present method gives a valuable reference to further study of the biogeochemical cycle and eco-effects of P in freshwater environment using δ¹⁸O_P analysis.