A new depositional model in droughty climate, named flood—overlakes, is built in this paper. Rivers of the province act as seasonal flood under droughty, most of the rivers fade away in the hungriness, river water was drained out in the way of surface vaporation, vegetation ecology consumption, underground seepage, terminal river—lakes and river overflowed lakes, so that no whole catchement center nor the lacustrine sediment were formed. Thus, the basin filling characters are clearly different from that of the classic lacustrine depression basins and that of the showllow lacustrine basins. During K2q4( the 4th Member, Quantou Fm., Lower Cretaceous) time, the Songliao Basin was characterized by drought climate, flat terrain, poor and monotonic biology varieties and quantity, and so on. There were five sendimentary systems and seven main river systems in the southern basin.Flood—overlakes in depression basins have the following features: ① no united catchment area could developed, as a result, widely distributed lacustrine and delta sediment are lack; ② the rivers act as seasonal flood, some of them disappear in the hungriness as the landform becomes flat, and fluvial facies is the main type of the basin filling; ③ the lacustrine basin bears many provinces and overlakes, and sandstone was depsited everywhere in the basin; ④ there are 3 types of relationship in the spatial distribution between the rivers and the overlakes: a river can passes through, passes around and passes away in the the overlakes. As a new type of depositional model in Mesozoic and Cenozoic, flood—overlake has important theoretical and practical sense.