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 A degrees 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.