The phase relations of TiOx containing systems in reducing atmospheres are extremely important for understanding the smelting process for vanadium titano-magnetite. In the present work, the equilibrium phase relations for the core CaO-SiO2-Ti3O5 system at 1400 °C and oxygen partial pressure of 10−16 atm were determined. Ti3O5 was confirmed as the stable Ti oxide in CO(g)-C equilibrium, and the solid phases Ti3O5, 3CaO·Ti3O5, SiO2, and CaO·SiO2 were found to coexist with the liquid oxide within the composition range investigated. The 1400 °C isothermal phase diagram of the CaO-SiO2-Ti3O5 system was constructed to demonstrate the areas of single liquid, as well as the two-phase and three-phase domains. Furthermore, a comparison with results in the literature indicated that the single liquid domain at 1400 °C greatly shrinks when the oxygen partial pressure decreases from air to 10−16 atm, and the corresponding liquid coexisting phases of TiO2 and CaO·TiO2 transform to Ti3O5 and 3CaO·Ti3O5.