The electrolyte is one of the most important components of vanadium redox flow battery (VRFB), and its stability and solubility determines the energy density of a VRFB. The performance of current positive electrolyte is limited by the low stability of VO2 + at a higher temperature. Phosphate is proved to be a very effective additive to improve the stability of VO2 +. Even though, the stabilizing mechanism is still not clear, which hinders the further development of VRFBs. In this paper, to clarify the effect of phosphate additive on the positive electrolyte stability, the hydration structures of VO2 + cations and the reaction mechanisms of precipitation with or without phosphate in the supporting electrolyte of H2SO4 solutions were investigated in detail based on calculations of electronic structure. The stable configurations of complexes were optimized at the B3LYP/6-311 + G(d,p) level of theory. The zero-point energies and Gibbs free energies for these complexes were further evaluated at the B3LYP/aug-cc-pVTZ level of theory. It shows that a structure of [VO2(H2O)2]+ surrounded by water molecules in H2SO4 solution can be formed at the room temperature. With the temperature rises, [VO2(H2O)2]+ will lose a proton and form the intermediate of VO(OH)3, and the further dehydration among VO(OH)3 molecules will create the precipitate of V2O5. When H3PO4 was added into electrolytes, the V-O-P bond-containing neutral compound could be formed through interaction between VO(OH)3 and H3PO4, and the activation energy of forming the V-O-P bond-containing neutral compound is about 7 kcal mol−1 lower than that of the VO(OH)3 dehydration, which could avoid the precipitation of V2O5 and improve the electrolyte stability.