The invention discloses a preparation method of potassium-magnesium fertilizer. The method comprises the following steps: crushing polyhalite and potassium feldspar as raw materials into powder, supplementing crystallized magnesium chloride, and preparing the potassium-magnesium fertilizer by roasting, soaking in water, filtering and evaporating. Compared with the prior art, the method has the advantages as follows: firstly, the polyhalite and the potassium feldspar are adopted as the raw materials, the raw materials are easily available and low in cost, potassium-containing ore widely existing in China is fully utilized to replace brine, and the technical problem of shortage of agricultural potassium resources in China is solved; secondly, elements such as potassium and magnesium in the raw materials are sufficiently utilized, and magnesium is further supplemented by combining added crystalline magnesium chloride, and the produced potassium-magnesium fertilizer provides potassium andmagnesium required for growth of crops and has good fertilizer effect; thirdly, the preparation method is simple, the production requirement is low, and the production cost is low, so that the methodis easy to popularize and use.