Abstract: Vanadium is a strategic metal in many countries, and it is mainly extracted from vanadium slag produced in titanomagnetite metallurgy. The traditional sodium roasting process for vanadium extraction poses environmental threats, and a green calcification process has been proposed. However, the vanadium extraction rate in the calcification process is much lower than in the sodium roasting process, which is related to vanadium solid solubility in Fe2TiO5. Previous studies about vanadium behavior in Fe2TiO5 were conducted in air, with a vanadium oxidation state of V5+. Vanadium with lower oxidation states has been detected in the tailings in the calcification process. The present paper studied the effects of vanadium oxidation states on the solid solubility in Fe2TiO5 through solid-state reaction, XRD characterization, TEM characterization, XPS analysis, and solid solution modeling. The relative interaction values between vanadium oxides and Fe2TiO5 are obtained as |"L" _("V" _"2" "O" _"3" ) |>|"L" _("V" _"2" "O" _"4" ) |>|"L" _("V" _"2" "O" _"5" ) |, indicating that vanadium with lower valence is preferable to be solid dissolved in Fe2TiO5. The results imply that insufficiently oxidized vanadium increases the vanadium content in the Fe2TiO5 phase during vanadium slag's calcification roasting. Besides, experimental conditions optimization shows that higher experimental temperature, vanadium introduction as V2O3, and a high-purity argon atmosphere would lead to higher vanadium solubility in Fe2TiO5, and high temperature is beneficial for the release of vanadium from vanadium-containing Fe2TiO5 when dissociated in air.