The quality of colon preparation plays a crucial role in colon cancer prevention, especially for right side colon. So far, colon cleansing is still not adequate in up to 10-30% colonoscopies and leads to increase risk of missed neoplasm.Recent literature on colonoscopy preparation revealed the dosage, timing and dietary restriction could influence colon preparation quality. Previous study demonstrated colon cleansing on the day of colonoscopy had better cleansing quality. However, the ideal colon preparation is not yet determined. the investigators herein consider the in-hospital setting of colon cleansing room can ensure better colon cleansing timing, colon preparation evaluation reliability and patient's compliance of colon cleansing. Therefore, the investigators design a prospective randomized controlled trial to compare cleansing quality between the setting of colon cleansing room and the standard colon preparation.