Skeleton growth has the annual, month and day rhythms, and the skeleton can record geological history as a paleontology clock through its growth rhythums. The discovery of annual density bands of mass coral skeleton had a profound effect on coral paleoclimatology, and many patterns of density bands have been revealed throughout the global tropic ocean since then. At the first stage, people tended to explore the control factors of density changes from the environment where the coral grows, but they did not succeed. Since later part of 1980s, some researchers have focused on the skeletal architecture and integrated the signals of both environment condition and skeleton density bands to reveal the mechanism of skeleton density changes, and finally a model of skeleton growth was developed which can explain the almost existed patterns of skeleton density changes. The thickness of coral soft tissue layer is a significant parameter because its variations relative to skeleton extensions affect the density patterns, but unfortunately, it has not been got enough attention even now. The relationship between skeleton calcification and photosynthesis of zooxanthellae is a controversial question, and the mechanism of calcification is still being explored.