Abstract:The systematic analytical method of grey theory was used to study the relationships between chlorophyll-b and 10 environmental factors (temperature, plankton biomass, salinity, dissolved oxygen (Do), pH, phosphate, nitrate, nitrite, Fe, and Mn) in seawater. Samples were collected from Dapeng Bay of the South China Sea, near Yan-tian area of the Shenzhen City. The resulting grey relative values and their permutation indicated that nitrate, phosphate, nitrite and pH had more influence on the chlorophyll-b concentration in seawater than any other factors．A grey model{GM(l，n)｝was established to relate chlorophyn-b concentration with four factors among the ten．