The confluences water of Qujiang-Jialing River and Fujiang-Jialing River was investigated in this paper for understanding the characteristics of phosphorus (P) forms and the effects of ultraviolet (UV) irradiation. Through analysis of P speciation, the results showed that the concentration of total P was above the Eutrophication threshold (0.2 mmol x m(-3)) in this study. Meanwhile, total dissolved P (TDP) and dissolved inorganic P (DIP) were the predominant forms of total P (TP) and TDP respectively at all sampling sites, which was independent of confluences. After confluences of Fujiang-Jialing River and Qujiang-Jialing River, the concentration of all forms of P in main stream increased, among which particulate P (PP) showed the greatest increase of up to 129%. Furthermore, water was fractionated by relative molecular weight (M(r)) of composition, and the results showed the predominant forms of dissolved P (DP, < 0.22 μm) was in the soluble high M(r) fraction (0.5 x 10(3)-10 x 10(3)), which was ranged from 47%-61% of DP. After confluences, no significant changes of P contents in different M(r) fraction of water were observed in the main stream (Jialing River). Moreover, UV irradiation experiment was conducted, and it suggested that P in free forms of water (< 0.5 x 10(3)) increased by 10% - 29%, as compared with the decreasing P in the other two M(r) fractions of M(r) 10 x 10(3)-0.22 μm and 0.5 x 10(3)-10 x 10(3). Additionally, in comparison with confluence site samples of Fujiang-Jialing River, the decreasing trend of P in colloid fraction (10 x 10(3)-0.22 μm) was higher in Qujiang-Jialing River. Thus, it confirmatively indicated that UV was an important environmental factor influencing the geochemical fate of P in aquatic system.