Taking the South Passage of Changjiang River mouth as an example, this article examined the extent and controlling factors of seasonal changes in the bed level of the river mouth channel bar. The correlation between the bed level and the riverine discharge was utilized to forecast the possible changes in the bed level under normal and extreme riverborn discharges.The bed level of the channel bar is sensitive to changes in riverine discharge and typhoons. The channel depth is negatively correlated with the river water discharge with a time lag of 1 to 2 months. The range of the change between flood season and dry season in particular years of minimal, normal and maximum river discharges was 0.4～0.5 m, 0.6～0.7 m and 0.9～1.0 m, respectively. An accretion of 0.4 m is likely to be generated by a ten-year-once typhoon.In case of a hundred-year-once storm superposing on an extremely high river flood, the seasonal change in bed level may probably reach 1.5 m.