Provided is a method for preparing manganese sulfate monohydrate, which comprises the following steps: (1) reacting MnO with (NH4)2SO4 solution under heat, and separating solid from liquid after reaction 2) determining the concentration of MnSO4 in the resulting solution of step 1), adding equimolar ammonium sulfide solution thereto to carry out reaction under agitation, separating solid from liquid after reaction, and then washing solid phase: 3) dissolving the solid phase with concentrated sulfuric acid, adding hydrogen peroxide to the obtained solution, heating to boiling, adjusting the pH value of the solution to 5-6, removing the solid phase through precise filtration, evaporating and oven drying the filtrate to obtain MnSO4.H2O product. The impurities are separated away and the amount of byproducts is reduced by this method, and the yield of manganese sulfate monohydrate is improved.