As for the application of ultrafiltration in water treatment process, one of the key points is the selection of suitable threshold flux. Based on the needs of ultrafiltration in water works, four kinds of water sources including raw water, water during coagulation, water after sedimentation and water after sand leach were used. The effects of different pretreatment on threshold flux obtained from flux cycling method were compared and analyzed. The irreversible threshold flux of raw water was 11 L/(m²·h), which was due to high organic maters and turbidity. The irreversible threshold flux of water after sand leach was higher than 55 L/(m²·h), which was similar to that of water after sedimentation. It is suggested that the ultrafiltration membrane system can be installed after sedimentation.