Recovering manganese resource from the mother liquor on one hand can improve the economic benefit of the whole process, and on the other hand is beneficial to realizing closed cycle of mother liquor and then can increase the environmental interest of the whole process because it can avoid the enrichment of Mn in the solution during recycling mother liquor which will bring side effect to the quality of vanadium pentoxide. Using oxalic acid to effectively separate the Mn2+ from the mother liquor was proposed and the effects of the solution pH, adding amount coefficient of oxalic acid, reaction temperature and time on the recovery rate of Mn and phases composition of the precipitations were investigated, and the effect of the numbers of recycling mother liquor treated by oxalic acid to recover manganese on the leaching process was studied as well. The results showed that oxalic acid can effectively separate the manganese from the mother liquor and the recovery rate of manganese reached 94.33% under the conditions that the solution pH was 4.0, adding amount coefficient of oxalic acid was 1.5, reaction temperature was 50℃ and the reaction time lasted for 60 min. The obtained product was hydrated manganese oxalate with purity of 98%, present as tabular crystals and formed in flower cluster-like. After treated with oxalic acid to remove manganese, the mother liquor was recycled to leach the roasted slag, and it was found that the recycling numbers have little effect on the vanadium leaching rate and the concentration of manganese contained in the vanadium-bearing solution, indicating the proposed method was helpful for realization of closed-cycle of the waste water in the whole process of vanadium extraction with calcified roasting and acid leaching.