The high Ti-bearing blast furnace slag is an important mineral resource which contains abundant TiO₂. In order to achieve the enrichment of TiO₂, the authors investigated the phase transitions and reaction mechanism of the blend of high Ti-bearing blast furnace slag and ammonium sulfate in the roasting process. The chemical calorific effect of the blend and the phase composition of high Ti-bearing blast furnace slag as well as the roasting products were characterized by means of TG-DTA and XRD. The results showed that the main mineral phases of the slag were perovskite, diopside and spinel, and there existed three endothermal valleys in the course of heating, which were caused by thermal decomposition of (NH₄)₂SO₄, reaction of the decomposition product with metal oxides in the slag and thermal decomposition of the intermediate product respectively. Ammonium sulfate was decomposed into (NH₄)₃H(SO₄)₂ under lower temperature, which reacted with the metal oxides in the perovskite and diopside to form CaSO₄. Al₂O₃ in the spinel, which reacted with (NH₄)₂SO₄ and (NH₄)₃H(SO₄)₂ to generate (NH₄)₃Al(SO₄)₃ at 300～375℃; at 375～425℃, (NH₄)₃ Al(SO₄)₃ reacted with Al₂O₃ to form NH₄Al(SO₄)₂, and then NH₄Al(SO₄)₂ was decomposed into Al₂(SO₄)₃ when the roasting temperature rose to 500℃.