Abstract: This paper use the balancing principle of input and output of inorganic nitrogen and inorganic phosphorus, based on the investigation and detection of the input quantity of inorganic nitrogen and inorganic phosphorus from seawater exchange, release of sediment, runoff of land and river, excretion of ocean animals in Dadeng Island sea area from 2000 to 2001. By investigating and detecting the input quantity of inorganic nitrogen and inorganic phosphorus from phytoplankton, algae, the quantity of inorganic nitrogen and inorganic phosphorus left for Porphyra and Laminaria were estimated, and then the carrying capacity of Porphyra and Laminaria were estimated by calculating the concentration of organic nitrogen and phosphorus. The result shows that in Dadeng Island sea area the carrying capacity of Porphyra and Laminaria estimated by inorganic nitrogen equity are 5.43104 and 32.84104 t, the density of unit area for culture are 7.21 and 43.61 thm-2, respectively. The carrying capacity of Porphyra and Laminaria estimated by inorganic phosphorus equity are 1.97104 and 12.22104t, the density of unit area for culture are 2.62 and 16.23 thm-2, respectively.