To reveal the effect on tidal flowing by main dynamic factors in the Modaomen estuary，we use a method of main tidal flux algorithm to calculate characteristics of tidal flowing under different time scales in half month of flood and dry seasons，and apply the method of multiple linear regression to fit their correlation.The results show that vertical averaged velocities during ebb period in Modaomen waterway in flood season is about 14% higher than those in dry season，and only about half of those during flood period.Residual velocities in the waterway during flood season are rather large and can be ignored during dry season.There is mutual collision of rising currents between the east and west branches outside the estuary of the waterway during flood season，while both point to the direction of southwest during dry season，and residual velocities in the west branch are more than those in the east constantly.Regardless of seasons，there is a counterclockwise semi-circular flowing structure close to seabed outside the estuary，more apparently during dry season.The impact of tidal power and wind on residual currents in the waterway are small，while runoff power takes position during flood season and ignored during dry season.The runoff power is strong during flood season，and the tidal fluxes of the east branch and the west branch are consistent.During dry season，net tidal flux from the waterway is mainly transported to the east branch，following southwest coastal current and entering the western branch，and then is exported towards the south of outer sea with offshore tidal current，which is consistent with the structural characteristics of the counterclockwise semi-circulation outside during dry season.