Abstract: This paper is devoted to the grain-size characteristics of a 550-cm-long sediment core, which was retrieved from the Lake Dayeze, a waterbody used to connect to the Lower Yellow River (LYR), with the purpose to reveal the environmental changes of the LYR during the last 1400a. The sediments are dated by high-precision AMS ¹⁴C dating. According to the grain size frequency distribution curve and cumulative probability curve of the sediments, in addition to lithological characteristics, the changes in depositional environment of the study area is divided to two phases: the 550~385cm composed of dark-grey clay and dark-grey caesious clay, belongs to a lacustrine sedimentary environment; the 385~30cm composed of yellow-brown silt and light yellow clay, is deposited in a fluvial environment. By Integration of grain size composition and grain size parameters, as well as historical documents, we inferred that the lake has experienced five distinct stages of environment evolution during the last 1400 years. There was a low level swamp environment during 625—940 AD, under dry and cold climate conditions in the study area. Then the lake level rose and the lake expanded due to flooding, under warm and wet climate conditions during 940—1250 AD, corresponding to the Medieval Warm Epoch. During 1250—1540 AD, it was characterized by strong hydrodynamic force under a fluvial environment and relatively humid climate, and then, the river hydrodynamic weakened, the climate became dry and cold again, corresponding to Little Ice Age. The channel of the LYR shifted back and discharged into Bohai Bay, by the 1855 AD flooding event. As a result, the study area was affected by the flooding events of the LYR again.