Keywords: 
• organism community /
• reef /
• evolution /
• sea level change /
• Lower Ordovician

Abstract: A lot of reefs have been discovered in Honghuavuan Formation of Lower Ordovician in the Central Yangtze Platform.Their rock comparisons contain two types.One is baffle stone.Another is binding stone.Their reef-building organisms contwin five types.They are hluegreen alga(containing blue bacterium and Girvanella),Galathium,BatostorrUc,Pulchrilamirya,and Archaeoscyphia as representation of rock sponge.They composite five types of reef-building organism community. Is hluegreen alga community(S-community),Galathium-hluegreen algae community(G S community),Archaeoscyphia- Galathium community(A-C community),Batostorruc-Galathium community(B-Q community),Batostotruc-Galathium-hlue-green algae c/iommunity(B-C-S community). Ecological environment of each of reef-building organism community and the most lithofacies assemblage of host rock may indicate characteristics of each communitv} which includes water body environment and the most depth range suitable.Each community can adapt normal shallow sea of warm clean rich sunshine and oxygen and cyclic better. But the most suitable depth may he different.S-community represents shallow water and small energy.C-S community and B-C-S com-munity may he 5-10 m and medium-large water energy.B-C community and A-C community may he 10-20 m and large water energy. Organism belt has been discovered in reefs of thickness over Sm in the study area. Organism belt is a result of organism community evolution and is the most intuitive evidence of organism community evolution.Organism belt within reefs from bottom to up may indicate two models of community evolution.The trend of their evolution is①A-C community or B-C com-munity→C-S community or B-C-S community→S-community②S-community→GS community or B-C-S community→A-Ccommunity or B-G community.This is an entire course of community from growth to death. Organism richness of community interior has also been changed with time. Because the most suitable water depth of each community is different the force of organism community evolution may he sea level change.According to the most suitable water depth of reef-building organic community detailed change of sea level can he diagnosed in reef-building term. Relative sea level change is from a falling course of 20-10 m→10-5 m→5 m to a rising of 5 m→10-5 m→20-10 m.The largest change magnitude of sea level is about 15 m.This is similar to the former. So, according to the reef-building organic community evolution can quantitatively study small magnitude sea level change.