The vertical distribution of marine phytoplankton is complicated. However, a bell-shape vertical profile of phytoplankton in mass concentration is frequently observed in coastal seas and open oceans. The corresponding Subsurface Chlorophyll Maximum Layers (SCML) have caused a very high scientific interest and have been studied well. SCM can be characterized by three parameters, i.e., depth, thickness and intensity of the SCML. However, challenges still exist on how to best characterize the three parameters. They are determined by marine hydrology, nutrient concentrations and phytoplankton species, etc. A few studies have been conducted to explore variability of three parameters among different oceans. In general, compared with coastal seas，seasonal variation of the three parameters in open oceans is insignificant. It is also indicated that the three parameters are deeper，thicker and smaller in open oceans than coastal seas. In oligotrophic ocean, the depth of SCML was mainly a function of light and turbulent diffusion. Its intensity was determined by light, turbulent diffusion and biogenic processes associated with zooplankton grazing. However, the quantitative relationship between these biogenic processes and the three parameters are not well established. For example, how the sinking rate of phytoplankton, physiological adaptation capabilities, motility, trophic interactions affect them are poorly understood, and further investigations are needed. This review also finds that the SCML study in coastal seas is rare, which could be due to the lack of a long-term and high-timeresolution observation in those areas. By use of satallite data, in situ data and physical-biological models, it might be useful for further understanding SCM furtherly.