Abstract: Tidal flats are located in sensitive areas of interaction between land and ocean, and their sedimentary environment changes are closely related to the evolution of coastal ecosystems and coastal development and utilization. In order to clearly understand the spatio-temporal variations of estuarine tidal flats, we selected the tidal flat of Langqi Island in the Minjiang River estuary as the study area, studied surficial sediment composition in different seasons, and discussed the spatio-temporal variations in sediment grain size distribution, providing a scientific basis for a deep understanding of the sedimentary process of the estuarine tidal flats in response to environmental changes. Results indicate that the surficial sediment composition of inter-tidal flat on Langqi Island is dominated by silt, with significant differences of sand and clay contents. The mean grain-size ranges from 2.2 Φ to 7.4 Φ, and shows a significant seasonal variations, with the average mean grain-size in summer being smaller than in winter. From west to east, the mean grain-size increases first and then decreases seaward, and the distribution area of coarse grain component in gradually increases from north to south. The distribution pattern of sediment on the inter-tidal flat of Langqi Island at the Minjiang River estuary is consistent with that of other estuaries, bays, and open tidal flats in the world, reflecting the result of the among sediment sources, hydrodynamics, and local geomorphology. However, due to significant differences in hydrodynamics, vegetation coverage and sediment supply in the study area, which cause significant seasonal changes in the sediment composition, the sediment grain-size compsition of the tidal flat in the Minjiang River estuary are very sensitive to environmental changes.