The eutrophication in coastal sea occurred mainly by the input of the nutrients from land,and increase the concentration of nutrients and change the ratio among the nutrients and often cause the appearance of phytoplankton bloom or redtide. When the bloom occurs the most energy is allocated the microbial loop, so bacteria reproduce intensively and decompose the organic matter and consume extensively dissolved oxygen, the anoxic environment appears and causes the extensive death of fish, shrimp and shell-fish for choke. The response of marine ecosystem to the eutrophication may be studied and measured quantitatively by the in situ mesocosm experiment. The mesocosm system is mass-conservative system, without the water exchange with the natural sea water, so the system become relatively simple and in which only ecological process occur. Based on the results of mesocosm experiment, the ecosystem model may be developed and validated. The mesocosm experiment is becoming the important tool to carry out research on the global marine ecosystem dynamics(GLOBEC).