In this paper, the phenomenon of filamentous branches hanging and their adjustment mechanism to the high light intensity for the free-floating green tide algae (Ulva prolifera) blooming in the Yellow Sea were studied.Yearly field surveys have been carried out throughout the occurring of green tide in the Yellow Sea during the past three years (2012-2014). The results showed that free-floating green algae can be divided into two parts—— floating air-sac and filamentous branches hanging under water.The floating air-sac bleached to death due to the stress of the high solar irradiance, whereas the filamentous branches could avoid specular adversity stress to survive. In July 2014, free-floating green algae were collected in the Qingdao Sea area, the air-sac and suspending filamentous branches were taken out for the measurement of chlorophyll fluorescence parameters, buoyancy and microscopic observation of cell structure. The floated air-sac Ulva thallus was white and green, and its F_v/F_m average value was about 0.29, concentration of chlorophyll was about 0.32 mg/g, with shrinking cytoplasm most skewed or discoloration. Suspending branches showed fresh green and its F_v/F_m value was relatively higher, the average was about 0.6, and the chlorophyll concentration was 0.87 mg/g, the thallus was filled with cell contents, and only a few cells began to shrink. Air-sac and suspending branches were separately cultivated at 25 ℃under 80 μmol/(m²·s) for a second observation a week later. Air-sac formed germ cell capsule and further formed the empty cells; suspending branches grew healthily and its shrinking cytoplasm ratio decreased after being cultivated.