Disclosed are an external plaster for treating children retention syndrome and a preparing method thereof. The plaster is formed by 9 kinds of traditional Chinese medicines, e.g., hovenia dulcis, rhizoma atractylodis, radix bupleuri, etc., with steps of decocting, concentrating, crushing, concocting and the like based on physiological and pathological features of children and etiology and pathogenesis of the children retention syndrome. According to the invention, the prepared external plaster has curative effects of strengthening spleen and improving appetite, promoting digestion and relieving stasis, regulating qi and guiding stagnancy, and moving the bowels, which mainly treats diseases such as no appetite for milk, abdominal fullness and distention, bellyache, emesis, constipation, crying and irritability, reddened tongue, thick or greasy tongue, or fever, dyspneic cough, phlegm(or wheezzing sound), etc., caused by the children retention syndrome. The invention is convenient and safe for using, with remarkable effect and good compliance of children, in addition, the composition facilitates to control the quality thereof and is suitable to industrial scale continuous production.