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Supercritical water gasification (SCWG) is famous for converting and utilizing sulfur-containing fuels without SOx emission. Under the hydrogen-rich reductive condition, sulfide is the prevalent inorganic existence form of sulfur with the presence of oxidative sulfurous compounds which is represented by sulfate. The transformation between sulfate and sulfide is of interest to realize regulating the final products. Thus, experimental and theoretical studies were applied in the present work to explore the unknown mechanism of thermochemical sulfate reduction (TSR) by H2. The degree of sulfate reduction by hydrogen decreases from 63 ± 5% of NaHSO4 to 24 ± 3% of Na2SO4, and 18 ± 1% of Na2SO4_K2SO4_NaOH mixture at the same reaction condition (600 °C, 25 MPa, 20 min). Sulfide, which presents in both liquid and gas phase, is the only reduction product, without any apparition of thiosulfate or SOx. Using SMD implicit continuum solvation model, the effect of supercritical water (SCW) on the reaction paths was detailed discussed in the density functional theory (DFT) calculations. The absence of SO2 in SCWG is a result of the solvation effect which guarantees the spontaneity of the consumption reaction of SO2. By comparing Gibbs free energy change and energy barrier of each reaction, the experimental results that higher sulfate reduction extent was reached under acidic condition was justified. The experimental and theoretical results are in good agreement with each other.