The invention discloses a method for preparing a high-purity manganese sulfate solution with low cost. The method comprises the following steps: (1) copper removal: obtaining copper-removed raw liquid manganese of which Cu is less than or equal to 0.2g/L; (2) pH value adjustment: adjusting the pH value of the copper-removed raw liquid manganese to be 1.5 to 5.0; (3) saponification: saponifying an extraction agent by using NaOH, Na2CO3, KOH, K2CO3 or ammonium hydroxide, so as to obtain sodium salt, potassium salt or ammonium salt containing the extraction agent; (4) manganese transforming saponification; (5) Mn and Ca extraction separation: making Ca in the raw liquid manganese enter an organic phase to be separated from Mn, and making all or most of Cu, Al, Zn and Cd enter the organic phase, so as to obtain extraction raffinate namely decalcified liquid manganese of which Ca is less than or equal to 5ppm; (6) deep purification: adding Na2S, NaHS, K2S, KHS or (NH4)2S into the decalcified liquid manganese, adjusting the pH value to be 4.0 to 6.0, and performing solid-liquid separation, so as to obtain a high-purity MnSO4 solution satisfying production of a positive precursor of a lithium ion battery.