The invention relates to the technical field of medical auxiliary medicines, in particular to a preparation method of bowel-clearing granules used before urological endoscopic surgery, which comprisesthe following steps: S1, weighing the following raw materials by weight: cassia seed, poria cocos, mulberry, hemp seed, bitter apricot seed, dried orange peel, white paeony root, celery, cabbage, banana, black fungus and Pu'er tea for later use; S2, pouring semen cassiae, poria cocos, mulberries, fructus cannabis, semen armeniacae amarae, pericarpium citri reticulatae and radix paeoniae alba intoa crusher, crushing, and sieving to obtain fine powder A for later use; S3, cutting celery, cabbages, bananas and black fungi into blocks, pouring the cut celery, cabbages, bananas and black fungi into a pot, performing cooking for 1-3 minutes, fishing out the cooked celery, performing cooling, squeezing juice, performing concentration for standby application, pouring dehydrated celery, cabbages,bananas and black fungi into a crushing machine, performing crushing, and performing screening so as to obtain fine powder B for standby application. According to the invention, excessive acidic substances in the metabolic process can be neutralized, toxins in blood and dust impurities remained in the digestive system of a human body are removed, and the effect of reducing blood fat is achieved.