The India-Eurasia collision led to the formation of an orthogonal collisional belt and an oblique collisional belt, which are the Tibet-Himalaya and the Sanjiang orogenic belts, respectively, and are now separated by the east Himalaya syntax. The Eocene Jianchuan basin is located in the southernmost region of the oblique collisional belt. Detailed sedimentary study of four profiles along the western margin of the Jianchuan basin revealed two stages of the basin infilling process, which were separated by the 37～34 Ma potassic volcanism. The earlier stage infillings were sourced from the southwest, and displayed upper and middle fan, and/then delta plain facies. The later stage infillings were derived from the northwest regions of the basin, which formed upper fan facies. Combined with the Eocene structures of the basin's basement, the authors hold that the Jianchuan basin is an Eocene foreland one. The Eocene Jianchuan foreland basin, along with its numerous equivalents in eastern and central Tibet, likely was previously located in the orthogonal collisional belt. The boundary between the orthogonal and oblique collisional belts might have been located in the south of the Jianchuan basin during the Eocene. The changes in tectonic location of the Jianchuan foreland basin indicate the successive northward growth of the oblique collisional belt due to the continuous northward indenting of the India continent into Eurasia.