The invention discloses a method for preparing a potassic-magnesian fertilizer. The method comprises the following steps: (1) mixing potassium chloride, mirabilite and high-magnesium salt lake brine according to a mass ratio of (1-2):(1-2):(5-15), stirring and reacting at the room temperature for 0.5-1.5 hours, and controlling the stirring rate to be 350-400 revolutions per minute, so that the raw materials are fully reacted and converted to obtain slurry; (2) adding the slurry obtained in the step (1) into flotation brine according to a mass ratio of 1:(4-6), adding a flotation reagent, performing two-refining one-sweeping flotation separation, washing, thereby obtaining a high-grade potassic-magnesian fertilizer and a mother solution; and (3) returning the mother solution to a salt pan, evaporating the mother solution to serve as high-magnesium salt lake brine, reacting with the mirabilite and potassium chloride, or directly taking the mother solution as the flotation brine to be used for flotation separation. The method disclosed by the invention is low in energy consumption, low in cost, simple in process flow, short in production period and convenient for industrial production, and the produced potassic-magnesian fertilizer is high in grade.