Due to the function of submerged dikes, part of water’s kinetic energy inverts to potential energy, which leads to backwater. The backwater height is hand in glove with the ratio of block-off area under different conditions, such as coefficient of roughness, bottom slope, profile shape and others. Combining the theoretical analysis with the flume experiment, we analyzed the backwater mechanism of submerged dike, and did the experiments on the affect of different bottom slopes, discharges, dams with various heights and dam structures on water level increasing and flow pattern, with the empirical relationship among water level increased, coefficient of local resistance, maximal flow velocity on top of dam and block off area established, and the correction factor of effective block off area for the natural river introduced. Based on the comparison between the calculated results and natural measured data, we verified the reasonableness of the empirical relation.