As the primary producers, acritarchs represent the base of the food chain in the Paleozoic marine ecosystem which links with the evolution of acritarchs. Based on high precision quantitative research, much information about Paleozoic marine ecosystem is provided. A quantitative analysis of Early-Middle Ordovician acritarch diversity changes in the Meitan Formation, Honghuayuan section, Tongzi, Guizhou is made and the the acritarch diversity curves are compared with sea level curves. We found that acritarch diversity changes were related to sea level changes during the Early-Middle Ordovician. Whereas sea level rose, and acritarch diversity also increased. An inshore-offshore model of acritarchs best explains the relative abundance changes of some acritarch taxa in relation to sea level changes. As the primary producers, acritarchs represent the base of the food chain in the Paleozoic marine ecosystem which links with the evolution of acritarchs. Based on high precision quantitative research, much information about Paleozoic marine ecosystem is provided. A quantitative analysis of Early-Middle Ordovician acritarch diversity changes in the Meitan Formation, Honghuayuan section, Tongzi, Guizhou is made and the the acritarch diversity curves are compared with sea level curves. We found that acritarch diversity changes were related to sea level changes during the Early-Middle Ordovician. Whereas sea level rose, and acritarch diversity also increased. An inshore-offshore model of acritarchs best explains the relative abundance changes of some acritarch taxa in relation to sea level changes.