The formation of mouth sand bar outside Modaomen Estuary of the Pearl River is subject to the influences of stream flow, tidal current, offshore current and wave. Besides, morphological condition and sediment transport are also key factors affecting the evolution of mouth sand bar. According to measured topographic data in the last four decades, this study is devoted to analyze the evolution of the central and western parts of mouth sand bar and its tidal channels. The morphological evolution in the mouth sand bar area is also investigated. Results indicated that 1) promoted by big flood events, the tidal channels on both sides of mouth sand bar extended and broadened stepwisely; 2) the eastern channel presented a certain swing trend, which was affected by explicit seasonal characteristic and wave effect; 3) the inner and outer slopes of central mouth sand bar were eroded, while the crest was deposited and heightened, with the north-south diameter shortened; 4) the Jiaobei sand bar on the west side was crescent-shaped due to the wave effect, and was prone to be scoured with channel down cutting by flood. The research result provides reference for the planning and rectification of the estuary.