Abstract: In this study, the leaching solution containing vanadium(IV), chromium(III) and iron(II) obtained in the process of extracting low valence vanadium from vanadium slag was taken as the research object, and the vacuum cooling method was applied to crystallize iron(II) from the leachatein the form of Mohr’s salt, which realized the efficient separation of iron from vanadium(IV) and chromium(III) and the preparation of Mohr’s salt. The effect of pH value, ammonium sulfate addition, reaction time, and crystallization time on the separation behavior of ferrovanadium crystallization were investigated. The results showed that the removal rate of iron reached 86.42% under the conditions of the vacuum degree of 0.08 MPa, pH value of 2.5, addition of ammonium sulfate (calculated as n((NH₄)₂SO₄)/n(FeSO₄)) of 1.2, reaction for 140 min and crystallization for 36 h. The loss of vanadium and chromium were only 0.52% and 1.64%, respectively. In addition, Mohr’s salt with a purity of 99.23% was obtained.