Abstract: The reefs (bioherm) or baffle reefs were discovered in upper Yingshan formation of the Middle Ordovician. The reef-building organism composes of Calathium, calcium algae, bryozoan, sponge, etc., dominated by Calathium. The reefs occur on the uplift of open platform, and were formed in regressive background without exposure signs and the division of reef front, reef behind and reef flank, but can be divided into reef base, reef core and reef cover. Reef base is dominated by echinoderm grainstone; reef core by Calathium bafflestone, algae-Calathium bafflestone, fractures, hole-groove filled substance, and reef cover by sparite calcarenite. The reefs undergo five stages of evolutions and develop two ecological communities.

Key words: Tarim basin, Yingshan formation, reef, sedimentary feature