H2SO4 concentration has an important influence on the performance of vanadium electrolytes and flow batteries. However, the comprehensive research is still inadequate. In this work, a series of electrolytes with 1.5 M vanadium ions (V2+, V3+, V3.5+, VO2+ and VO2 +) in various concentrations of H2SO4 (1.5 M, 2.0 M, 2.5 M, 3.0 M, 3.5 M and 4.0 M) are prepared and their physicochemical properties, electrochemical activities, single-cell performances, and static stability are studied in detail. The conductivity and viscosity of the electrolytes increase with increasing H2SO4 concentration. Besides, the electrochemical properties are improved in higher concentration of H2SO4 except for the charge transfer resistance and hydrogen evaluation reaction. Meanwhile, the single-cell performances with 3.0–4.0 M of H2SO4 are superior to those with 1.5–2.5 M of H2SO4. However, V3+ in 4.0 M H2SO4 is not stable enough to store for a long time at room temperature. The above results indicate that 3.0 M and 3.5 M of H2SO4 should be used as supporting electrolytes to achieve efficient and stable vanadium flow batteries. This work may also provide a guide for study and practical application of vanadium flow batteries.