The Late Triassic Carnian reef-oolite bank complex belongs to the Maantang(Hanwang)Formation， and is distributed along the margin of present Longmen Mountains in Mianzhu， Anxian and Jiangyou districts of northwestern Sichuan Province. It was discovered by the geologic mapping group led by the author in 1975. By microfacies analysis of reefs in this area， and comparison with similar reefs in the world， the results demonstrate that the reefs were built up owing to both baffling function of siliceous sponges mainly of hexactinellids and binding function of thrombolites formed mainly by cyanobacteria. Hence the reefs are of deep water reef mounds of bindingbaffling type， and of limemud matrix-supported reefs. By microfacies analysis， especially comparison with standard microfacies of rimmed shelf model and ramp model of the world， the author considers that the foundation for reef cluster growth(the oolitic bank and bioclastic bank)was formed not only on a ramp， but also on a homoclinal ramp. The inference of the Carnian homoclinal ramp in northwestern Sichuan Province， the installation of reef window model， and the compilation of sedimentary facies and palaeogeography maps serve as grounds for understanding geotectonic framework and sedimentary environments of the region.