The influencing mechanism of oxygen volume percent (%VO2) on the acid solubility of the titanium slag during the oxidization at 1273 K (1000 °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 (3 vol% O2), 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 6 vol% O2), 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 (21 vol% O2), 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.