The goal of this clinical trial is to evaluate the efficacy, safety and tolerability of lactulose in bowel preparation in IBD patients. The main question it aims to answer is: Does lactulose have better efficacy, safety and tolerability than 3L-PEG? Participants will be divided into PEG group or Lactulose group at a 1:1 ratio by a random number method. Each patient will get a leaflet provided by hospital introducing bowel preparation methods and diet restriction before bowel preparation. They will have bowel preparation with different drugs according to group. The grade of bowel cleansing will be assessed through the Boston Bowel Preparation Scale (BBPS). The tolerability, satisfaction and safety of the two bowel preparation methods will be assessed through the patients self-administered questionnaires.

Inclusion Criteria:

• All patients with confirmed IBD or suspected IBD for at least 3 months, aged between 16-75 years and requiring a colonoscopy are invited to participate in our study. The diagnosis of IBD is based on recognized standards, including clinical symptoms, endoscopy, radiology, pathology and surgical history.

Exclusion Criteria:

• Patients are excluded if they (1) are clinical active UC and CD, (2) are known or suspected gastrointestinal obstruction, bowel perforation, toxic colitis or megacolon, recent or active gastrointestinal bleeding, (3) are congestive heart failure, severe renal failure, (4) are severe dehydration or electrolyte disturbances, (5) are pregnant, (6) are allergic to PEG or Lactulose, (7) are diabetics, (8) are lactose or galactose intolerance, (9) refuse to participate in this study.