Abstract: Based on field outcrop observation and microscopic thin-section identification, the sedimentary characteristics and reef formation model of paleo-reefs in the Carboniferous Heshlafu Formation at the Altash section in the southwestern margin of Tarim Basin were comprehensively analyzed. Results show that the paleo-reefs are spot reefs formed in the open platform, and were divided into high-raised type and low-raised type. The reef-building organisms are abundant corals with a wide variety of reef-dwelling organisms, including brachiopods, gastropods, crinoids, and fusulinids. The reef grew in an uplift in geomorphology, and its base, core, cap, and other parts are clear. Based on the characteristics of reef development, geomorphological evolution, and sea level fluctuation in the study area, we concluded that the high-raised point reef grew on the top of a table, exposed and then died out with the sea level decrease, as indicated by coral skeleton limestone and bioclastic granular limestone. The low-raised point reefs developed in a relatively low flat with table, and were drowned when sea level rose, as indicated by thinly banded and wide lateral distributed coral skeleton limestone and bioclastic micritic limestone.