Abstract: On the basis of the structural setting of the Chengdu basin （plain）, base- ment configuration and structural features of the plain, stratigraphic position of the basement, absence of some geologic ages, relations of sediment sources, depositional environments, lithofacies changes and changes in depositional scope since the Late Cretaceous, this paper deals with the conditions, origin and age of formation of the Chengdu basin, puts forwards new recognition on the age of the "Dayi Conglomerate" and the lower boundary of the Quaternary, discusses neotectonic movements, and summarizes the history of geological evolution sincep the Late Cretaceous at the western margin of the Sichuan basin.