The invention relates to a method for preparing a high purity level lithium hydroxide monohydrate and belongs to the technical field of lithium hydroxide preparations. The method includes the following steps of adding calcium hydroxide into a lithium sulfate leaching liquid to adjust the potential of hydrogen (pH) value, filtering to obtain a lithium sulfate primary cleaning liquid, adding sodium hydroxide, then filtering and removing impurities, crystallizing to obtain a sodium sulfate decahydrate solid and a lithium hydroxide refrigerating fluid, finely filtering the lithium hydroxide refrigerating fluid, then performing an evaporating separation to obtain a Lithium hydroxide monohydrate primary crude product, dissolving the crude product by adding water, adding a refining agent to remove sodium, then performing an evaporation concentration, separating and leaching to obtain a Lithium hydroxide monohydrate secondary crude product, adding water into the secondary crude product, performing the evaporation concentration, cooling to crystallize, and drying to obtain the high purity level lithium hydroxide monohydrate. By means of the method, the evaporation liquid-solid ratio is reasonably controlled, and processes of crystallizing and crystal growing are modified, so that the high purity level lithium hydroxide monohydrate product is obtained, the process is simple, the operation is easy, the equipment investment is few, the product cost is low, the lithium recovery rate is high, product qualities are stable, and the method is suitable for the industrialized production of the high purity level lithium hydroxide monohydrate.