The existence of electrolytes in aquatic environment on the photocatalytic performance and coagulation of nanodispersed TiO2 hydrosol and the corresponding photocatalytic alteration were investigated by studying cations (Na+, K+, Ca 2+, Mg2+, and Al3+). The photocatalysis reactions of nano TiO2 with different dosages of electrolytes were measured by monitoring the degradation of Rhodamine B (RhB) under ultraviolet A (UV-A) irradiation over time. The results showed that the photocatalytic performance of TiO2 was improved by the presence of Al3+, while the performance was impaired by the other tested cations. The negative influences of divalent ions on the photocatalytic performance of TiO2 were more significant than monovalent ions. The TiO2 sol dispersed stable at nano scale at low concentration of electrolyte (< 0.01 mol/L) with slight change of pH, and coagulated into micro sizes at high concentration of electrolytes (> 0.1 mol/L) with larger increase or decrease of pH. The positive effects of Al3+ on the photodegradation rate of RhB might relate to the strong hydrolytic action of Al3+ in aquatic solutions. The photocatalytic processes of TiO2 in the presence of all ions followed the Langmuir-Hinshelwood model, and the reaction kinetic constant was increased with the decrease of pH caused by different cations. These work suggested a new perspective about the relationship between coagulation and photocatalytic performance of TiO2 hydrosols in electrolyte with hydrolysable cations, which demonstrated that TiO2 hydrosols may be suitable as photocatalysts in aquatic environments.