The dependence of the reduction behaviors of the M3O5-type (Fe2TiO5)a(MgTi2O5)b on the ratio of Fe2TiO5 and MgTi2O5 was investigated in a CO-CO2 gas mixture at 1123 K (850 °C), and the phase transformation mechanism was revealed. The reduction process of (Fe2TiO5)a(MgTi2O5)b underwent the formation of the intermediate (FeTi2O5)e(MgTi2O5)f in the same M3O5-type, and subsequently this intermediate further decomposed into the M2O3 phase. The a/b ratio had a marginal effect on its reduction of Fe3+→Fe2+ but largely influenced its phase transformation. To obtain a complete phase transformation into M2O3,a/b ≥ 0.9 is required, below which a considerable amount of inadequately decomposed intermediate M3O5 phase remains. A principle of suitable acid leaching condition for titanium slags with different a/b ratios was proposed accordingly, and an optimized plan for the upgrading process of low-grade titanium slags was recommended.