The invention relates to medicine for treating diarrhoea and a preparation method thereof and belongs to the field of traditional Chinese medicine. Diarrhoea, commonly known as 'having loose bowels', is a common symptom and refers to the situation that defecation times obviously exceed the frequency in ordinary days, the excrement is thin and contains more water, the defecation amount per day exceeds 200 g, or indigestible food or pus blood or mucus is contained, and diarrhoea is often accompanied with such symptoms as urgency sense for defecation, anus discomform and incontinence. The medicine is prepared by selecting and combining mantis egg cases, mangnolia officinalis, hyacinth beans, agastache rugosus, eupatorium fortunei, Chinese yams, fructus amomi, gordon euryale seeds, amomum cardamomum, dogwood, alpinia katsumadai, lotus seeds, amomum tsao-ko, pericarpium papaveris, dark plums, limonite, Chinese gall, red halloysite, light wheat, cortex ailanthi, white atractylodes rhizome, pericarpium granati, terminaliachebuia, astragalus membranaceus, nutmeg, atractylodes lancea, cinnamon, dried ginger, raspberries, cuttle bone, glutinous rice root hair, Chinese ephedra roots and cherokee rose fruits. All the medicinal raw materials are combined to achieve a synergistic effect in efficacy, and thereby diarrhoea can be effectively treated.