Abstract: The Late Triassic Longmenshan foreland basin developed as a flexural foredeep on the Yangtze passive continental margin during Indosinian orogeny, spanning the time period ca. 230~210Myr. The basin fill overlying a basal unconformity is a tectonic sequence bounded by unconformities, including Maantang Formation, Xiaotangzi Formation and Xujiahe Formation, and can be divided into four sequences (tectonostratigraphic units) by unconformities and flooding surface. The cross-sectional geometry of the foreland basin fill is typified by the wedge-shaped body, but it can be divided into wedge-shaped sequence and tabular shaped sequence. The unconformity between overlying Tectonic sequence of the Late Triassic Longmenshan foreland basin and underlying tectonic sequence of Paleozoic-Middle Triassic passive continental margin belongs to the basal unconformity of the Longmenshan foreland basin and is a typical flexural forebulge unconformity which marks the transition from the passive continental margin to the foreland basin in the western Yangtze plate. Accurately the basal unconformity locating between the Upper Triassic Maantang Formation and the Middle Triassic Leikoupo Formation belongs to the parallel or angular unconformity. On the surface of the basal unconformity, there are erosion pits, paleo-karst (including solution groove, cave and breccias), crust of paleo-weathering (including limonite, clay and quartz), and basal conglomerate (such as fine chert conglomerate). And, directing to the southeast, the basal unconformity continually truncated underlying strata, rifely developing as karst weathering surface. The overlying Late Triassic strata overlap to the southeast along the unconformity; the striking of overlapping lines, erosion belt and sedimentary faces are roughly parallel to the striking of the Longmenshan thrust belt, which shows the transiting from conformity to unconformity and the southeastward migrating with the advance of the thrust wedge from NW to SE. The basal unconformity shows establishment of the Longmenshan foreland basin and drowning of a distal margin of the Yangtze plate by carbonate ramp and sponge build-up, deepening into offshore marine muds, followed by progradation of marginal marine siliciclastics. The formation of the basal unconformity and uplift of the forebulge is driven by the flexural deflection of tectonic loads in the margin of the Yangtze plate. The orogenic load system initially advanced towards the Yangtze plate in the Carnian, associated with the rapid closure of the Songpan-Ganzi Ocean. Based on the weathering crust, the residual thickness, the missing strata, the erosion thickness, stratigraphic overlap in the Late Triassic on the basal unconformity, the migration rate of the basal unconformity, migration rate of forebulge, stratigrapic onlap rate, and erosion rate of forebulge have been calculated here, the results showed that the migration rate of the basal unconformity, migration rate of forebulge and stratigrapic onlap rate are all between 3mm·a-1 and 18mm·a-1, winch are similar to the thrust wedge advancing rate (5~15mm·a-1). Therefore, the thrust wedge advancing rate can be instead of the migration rate of the basal unconformity, migration rate of forebulge and stratigrapic onlap rate. However, erosion rate of forebulge is only 1/100 of thrust wedge advancing rate (0.02~0.08mm·a-1).