The Chengdu basin is located at the juncture of the Sichuan basin and theLongmen Mountains on the eastern margin of the western Sichuan plateau. Theformation and evolution of the basin were closely related to the uplift of theQinghai-Tibet plateau and the subsidence of the Sichuan basin. This paper discus-ses the geologic setting for the formation of the Chengdu basin and the evolution-ary feature of its paleogeographic environments. It is considered that the Cheng-du basin is the tectonic remains resulting from the disintegration of the LongmenMountains and Sichuan basin and that the early deposits--the Dayi conglomeratebed--belong to a post-tectonic piedmont alluvial--diluvial molasse formation, whichis notably different from pre--Quaternary deposits in lithology and structraldistribution. It is suggested that the Chengdu basin is the tectonic remains ofthe third phase of the Himalayan movement. This phase of the Himalayan move-ment laid the foundation for the Chengdu basin. The Dayi conglomerate was depos-ited as a result of energy balancc and compensation of this phase of orogen.Then the basin was sinking intermittently. From the beginning of the accumula-tion of the Dayi conglomerate to the Holocene, a maximum thickness of 540 mof coarse fragments (gravel and pebble layer) was accumulated in the basin. Sothe base of the Dayi conglomerate is defined as the lower boundary of the Qua-ternary tentatively.