The influencing mechanism of oxygen volume percent (%VO2) on the acid solubility of the titanium slag during the oxidization at 1273K (1000 degrees C) are investigated. %VO2 has marginal effect on the phase composition evolution, but greatly affects the microstructure evolutions during the oxidation, that is, increasing %VO2 promotes Fe outward migration and further results in the formation of pores and Fe-deficient M3O5 phase, which have opposite effects on the slag solubility. Particularly, for the slag oxidized at low %VO2 (3vol%O-2), its acid solubility is bad, due to no obvious Fe outward migration happens and particles show extremely dense structure, unfavorable for the subsequent reactions. For the slag oxidized at moderate %VO2 (around 6vol%O-2), its acid solubility can be promoted, since Fe outward migration slightly happens and pores form from the original dense structure. However, for the slag oxidized at high %VO2 (21vol%O-2), since excessive Fe outward migration results in the formation of Fe-deficient M3O5 phase in the particle center, which is hard to be reduced and further leached, its acid solubility is also bad, although more pores form in such cases.