Abstract: Branched and sac-like Ulva linza is first reported in this paper.Branched U.linza collected from Jiangsu, Shandong and other places,was researched.The results showed that there are two kinds of branches,one is real and the other is false.Life history of branched U.linza was also studied.Its life history is typically an isomorphic alternation of generations,in which the unisexual haploid gametophytes and diploid sporophytes alternate mutually.The thalli of U.linza collected from green tide outbreak in 2008 were branched and had sac-like structure which contributed to floating and growing on the sea.Free floating green tide algae became the substrate which the spores were attached to.A great quantity of spores attached to the sac-like thalli to grow.This could be one of the causes that the free floating seaweeds bloomed in Huanghai Sea area.