The invention discloses a method of preparing a potassium-magnesium fertilizer. The method comprises the following steps of by taking polyhalite and potash feldspar as raw materials, crushing the rawmaterials into powder, then replenishing magnesium sulfate, and performing calcination, water immersion, filtering and evaporation to prepare the potassium-magnesium fertilizer. Compared with the prior art, the method has the advantages that, firstly, by adopting the polyhalite and the potash feldspar as raw materials, the raw materials are available, the cost is low, potassium-bearing minerals which widely exist in China are fully utilized to replace brine, and the technical problem of shortage of agricultural potassium resources in China is solved; secondly, by fully utilizing potassium andmagnesium elements in the raw materials and combining with external the magnesium sulfate for further replenishment of the magnesium elements, the produced potassium-magnesium fertilizer provides thepotassium and magnesium elements required for growth of crops and is good in fertilizer efficiency; and thirdly, the preparation method is simple and is low in production requirement and production cost and liable in popularization use.