The invention relates to a method for processing watermelon frost; the method is realized by the following steps: 1. the step of preparing mirabilite crystal by extracting saltcake is as follows: saltcake is taken and added with hot water for dissolving, and then filtered, the filtrate is added with turnip strips or slices, boiled for 30 minutes, filtered, and placed in the refrigeration block ofa refrigerator, or placed at a place which is in line with sanitary requirement and has the temperature of 0-20 DEG C; about one hour later, the crystal is separated out and then is taken out ready for use; saltcake impurities are removed; 2. watermelon juice is prepared; 3. the mixed liquor of watermelon juice and mirabilite crystal is placed in the refrigeration block of the refrigerator, or placed at a place which is in line with sanitary requirement and has the temperature of 0-20 DEG C; the mixed liquor of watermelon juice and mirabilite is filled in a container which is placed in the refrigeration block of the refrigerator, or placed at a place which is in line with sanitary requirement and has the temperature of 0-20 DEG C for separating frost, the separated frost which has high water content is heated by a drying box and a water bath and dried; after crystallization water is removed, the watermelon frost is obtained; the method of the invention has the advantages of low toxicity and high purity of prepared frost, short period for preparing frost, high frost yield, no change of drug property, etc.