The Songpan- Ganzi orogenic belt is an important part of the northeastern Qinghai- Tibet Plateau. It is the major convergent region of the North China, the Yangtze and the Qiangtang blocks. The orogenic belt consists of Mesozoic low- grade metamorphic strata and a series of magmatic rocks, and has recorded convergent events of the three blocks since the Indosinian Period. Yajiang remnant basin within the orogenic belt, in which an extremely thick sedimentary series and widespread magmatic rocks occur, is an ideal region for research on the tectonic evolution of the Songpan- Ganzi orogenic belt. This study undertakes detrital zircon LA- ICP- MA U- Pb geochronology. The results show that the zircon U- Pb age values focus on four peaks, 231~281 Ma, 424~502 Ma, 707~983 Ma, 1539~1850 Ma, respectively, indicating that the west margin of the Yangtze craton and the Songpan- Ganzi orogenic belt have been subjected to at least four stages of intensive tectonic- magmatic events that were recorded in the Triassic strata in this area. The zircons with ages 231~281 Ma are most probably from the arc granites of the east Kunlun Mountains, formed during subduction of the Permian Songpan- Ganzi Ocean downward to the North China block. The zircons with ages 424~502 Ma were derived from the Yangtze block, representing a splicing event of the south Qinling, the north Qinling and the North China blocks. The zircons with ages 722~983 Ma are from the Yangtze block, and most probably from the granites of south Qinling formed during the subduction of the Neoproterozoic extensional upper Yangtze craton basin below the North China block. The granites are the products of crustal growth event in the Jinning period. The ages 1539~1850 Ma correspond to that of basement of the North China block, when the eastern and the western parts of the North China craton collided in the central area and the North China paleo- continent was in the process of further solidification and extension. The Archean material and newly generated igneous and sedimentary rocks were inherited, and a great many of these detrital materials were carried into the Songpan- Ganzi basin through an east- west directed transport system during the closure and collisional orogeny of the south Qinling Ocean in Middle- Late Triassic Epoch. Our results suggest that the origin of the Triassic Xinduqiao Formation in the Songpan- Ganzi flysch basin was mainly the material from the east Kunlun Mountains, the North China block and the Qinling orogenic belt. The youngest detrital zircons can limit the oldest depositional age. The weighted average age of four youngest zircons is 241. 8±4. 5 Ma, temporally constraining the deposition event of sedimentary rocks.