The invention provides a method for preparing potassium chloride from a solid ore containing potassium, ammonium and magnesium. The solid ore contains the following components in percentage by weight:5.1%-5.9% of potassium ions, 1.0%-1.4% of ammonium ions, 9.5%-10.0% of magnesium ions and 35.2%-37.1% of chloride ions. The method comprises the following steps: (S1) crushing the solid ore containing potassium, ammonium and magnesium, and sieving, so as to obtain a first screen underflow; (S2) mixing the first screen underflow with water in a mass ratio of (100) to (50-70) to adequately decomposing the first screen underflow, carrying out wet sieving by virtue of a 40-80-mesh sieve, and carrying out solid-liquid separation, so as to obtain a second screen underflow; and (S3) carrying out primary flotation on the second screen underflow so as to obtain a first flotation concentrate, and carrying out washing, solid-liquid separation and drying, so as to obtain the potassium chloride product. According to the method, a decomposing-sieving-floatation process is formulated according to the constitution of the solid ore containing potassium, ammonium and magnesium and is utilized for preparing potassium chloride, obtained potassium chloride can be used as a production raw material, the whole process flow is simple and low in cost, the utilization problem of potassium is solved, and theresource utilization rate is effectively increased.