The backwater caused by bridge piers on plain rivers was numerically simulated using the water-blocking ratio of piers(α)as the control parameter. The backwater effect of piers was investigated in terms of the creep height in front of piers and the backwater height in front of a bridge according to different generation mechanisms and influence areas. The influence area of the creep height in front of piers was determined by the variation coefficient of the cross-sectional backwater height. It is concentrated around the piers with higher water-blocking ratios(α>7%). The relationships between the dimensionless relative backwater height β_yg, the relative distance from the location with the maximum backwater height to the bridge pier, and the water-blocking ratio were analyzed using the cross-sectional average backwater height as the representation variable. The results show that the maximum backwater height occurs at four to five times the pier width in front of the pier with higher water-blocking ratios, and the increase in the water-blocking ratio causes the maximum backwater height in front of the bridge to rapidly increase and the backwater area to continuously enlarge. The water-blocking ratio of 7% is a significant demarcation value for the backwater height in front of bridges on plain rivers. Thus, the water-blocking ratio of medium and large bridges should not be more than 7%.