Flood overlake sedimentary system was formed in the shallow lake with flat paleotopography in the early stage of lacustrine basin development under the condition of arid and semi-arid climate. And near-source paroxysmal flood deposits, which is different from the common lacustrine fluvial delta system, is often developed in this environment. The target layer of this study is the lower part of the fourth member of Paleogene Shahejie Formationin in Dongying Sag, Bohai Bay Basin, Eastern China. Core, well logging, 3D seismic and geochemical data analysis are utilized to study the flood overlake sedimentary characteristics of shallow lakes under alternating wet and dry climates. The results show that: 1) The target layer develops the iconic red layer, which is the result of the alternation of humid short-term flood period and dry long-term flood intermittent period under the condition of dry climate; 2) Two different depositional environments and climatic conditions are identified in the target layer, which can be divided into two parts. The lower part of the target layer was formed in the flood intermittent period, indicating arid climate, resulting in decline of lake level and reduction of the lacustrine basin scale, which manifests a high-frequency turbulent lacustrine basin in the relatively low water level period, developing flood overlake deposites, mainly including flood overlake sand flat, sand-mud mixed flat and mud flat. The upper part of the target layer was deposited in flood period, with humid climate, rising lake level and expanding lake basin. It was a high-frequency turbulent lacustrine basin during the relatively highwater level. Shallow delta and shore-shallow lake were deposited successively in the upper part of the target layer, the main sedimentary types of which are underwater distributary channels, interdistributary bays and beach bars. Based on the understanding, the deposition model of flood overlake deposition has been established, including two deposition models of flood period and flood intermittent period. Under the guidance of the models, the distribution of sedimentary facies in the study area was characterized by using drilling and 3D seismic data, which provides geological support for oil and gas exploration in the study area.