The invention discloses cold tea for treating oral ulcer and a preparation method thereof. The cold tea comprises, by weight part, polygonum capitatum, beonninghausenia sessilicarpa, sheepear inula herb, chrysanthemum, dandelion, Chinese mosla herb, licorice root, siraitia grosvenorii, root of kudzu vine, chestnut, dark plum, unprocessed rehmannia root, tomato, spinach, lotus root, green plum, mung bean, dried orange peel and a sweetening agent. The preparation method comprises the following steps of 1, tomato, spinach and mung bean juice preparation, 2, traditional Chinese medicine raw material pretreatment, 3, chestnut and lotus root powder preparation, 4, extraction refining, 5, extract preparation, 6, preparation, and 7, sterilization and cooling. The preparation method adopts a distillation method and guarantees maximizing of raw material medicinal values and nutrition values. The cold tea can treat oral ulcer and has effects of clearing heat and removing toxins, dispelling summerheat and moisturizing dryness, and promoting the production of body fluid to quench thirst.