Abstract: The effect of Hizikia fusiforme multiplication on marine environment factors of large seawall on coastal waters in Yunbo Bay was studied. The result showed that Hizikia fusiforme was suit for uptaking N and P. In the aquaculture period of Hizikia fusiforme during Feb. to Apr. 2011, COD of fillet area was decreased from 140.3 mg/L to 65.7 mg/L, by 53.2%; NH4+-N was decreased from 0.72 mg/L to 0.53 mg/L, by 26.4%; Phosphorus was decreased from 0.35 mg/L to 0.17 mg/L, by 51.4%, variation of chlorophyll was not obvious, which indicated that Hizikia fusiforme played an important role in eutrophication of coastal waters..