Among the eight estuary outlets at Pearl River Estuary, the Modaomen is the largest passageway for flood relief with a strong dynamic mechanism of runoff. It has complex tidal currents and wave dynamics with the influences of circulation and monsoon, resulting in various shapes and evolutionary characteristics of the mouth bar. This paper analyses the dynamic characteristics of the Modaomen Outlet and the recent evolutionary patterns of its mouth bar. The study indicates: (1) In draught seasons (from October to March), the spring tides outside the mouth are usually weak, while the ebb tides in the southwest direction are often strong; But in flood seasons (from April to September), the spring tidal currents flow along the north-east side, while the ebb tidal currents flow in the southwest direction; (2) In draught seasons, there are stable alongshore currents in the southwest direction in the sea area outside the Modaomen, which is mainly controlled by the Northeast Monsoon of the South China Sea. In flood seasons, the directions of the alongshore currents are various, which are mainly influenced by the instability of runoff volumes and monsoons; (3) the inner slope of the bar moves southward due to the influences of the flood runoff and the external slope moves northward due to the influences of the waves on lifting the sands and the influences of the alongshore currents on sediment transport. In addition, the top of the mouth bar are heightened due to sedimentation and the south-north diameter shortens. Thus, in the inner part of the Modaomen, the N-direction ebb tides gradually merge into the SW-direction alongshore currents, and the mouth bar grow mainly in the southwest direction.