The invention belongs to improvement on a top-grade potassium magnesium sulfate fertilizer preparation process, and particularly relates to a method of using carnallite mine and potassium mixed salt mine to prepare top-grade potassium magnesium sulfate fertilizer. The method includes: (1), decomposing, and floating; (2), ball-milling, converting, and floating; (3), washing, and separating; (4), converting, and filtering; (5), drying. Crude potassium chloride can improve product grade, so that high-grade potassium magnesium vitriol products can be obtained; the method is simple in principle, short in process route and easy to operate, and recovery rate of K+ reaches higher than 70%.