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As the byproduct in the smelting process of vanadium titano-magnetite, titanium-bearing blast furnace slag (TBFS) can be converted to a titanyl sulfate (TiOSO4) solution containing MgSO4 and Al2(SO4)3 impurities via dissociation by concentrated H2SO4 (80–95%) at 80–200 °C, followed by leaching with H2O at 60–85 °C. In this study, hydrated TiO2 was prepared by hydrothermal hydrolysis of a Mg/Al-bearing TiOSO4 solution at 120 °C and the hydrolysis law was investigated. The experimental results indicate that MgSO4 and Al2(SO4)3 accelerated the hydrolysis and significantly affected the particle size (increasing the primary agglomerate size from 40 to 140 nm) and dispersion (reducing the aggregate size from 12.4 to 1.5 μm) of hydrated TiO2. A thermodynamic equilibrium calculation showed TiOSO4 existed as TiO2+ and SO42− in the solution, and MgSO4 and Al2(SO4)3 led to little change of [TiO2+], but an obvious decrease of [H+], which favored the hydrolysis process. At the same time, the coordination–dissociation mechanism of SO42− and Al(SO4)2− facilitated the lap bonding of Ti-O-Ti, promoting the growth of hydrated TiO2 synergistically.