In this paper, for laying the foundation in future for the forecast and potential evaluation of green tide occurrence in the Yellow Sea, the photosynthetic physiological characteristics parameters and grade evaluation of free floating algae,Ulva prolifera drifting along Qingdao coast were studied. According to the difference of algae physiological states corresponding to the maximum light quantum yield (Fv/Fm), the floating Ulva proliferaare divided into four categories: Grade I: green healthy algae (G), the range of Fv/Fm value is from 0.70 to 0.75; Grade Ⅱ: green yellow sub health algae (GY), the range of Fv/Fm value is from 0.60 to 0.69; Grade Ⅲ: yellow ill health algae (Y), the range of Fv/Fm value is from 0.41 to 0.59; Grade IV: white decayed algae (W), the range of Fv/Fm value is from 0 to 0.40. The free floating algae sampled at various sampling points along Qingdao coast in 2011 can be classified into the following three categories after the measurement and evaluation on the algae state: Grade Ⅱ, the highest Fv/Fm value was 0.68 corresponding to the total chlorophyll content was (0.608±0.032) mg/g; Grade Ⅲ, the highest Fv/Fm value was 0.57 corresponding to the total chlorophyll content was (0.226±0.031) mg/g; Grade IV, the highest Fv/Fm value was 0.29 corresponding to the total chlorophyll content was (0.088±0.029) mg/g. Furthermore, the healthy levels of floating thalli were mainly composed of Grade Ⅲ (51.34% 71.37%) and Grade IV (17.84% 28.08%) in QD1 3 sampling points, and were predominated by Grade Ⅲ (above 95.13%) in QD4 6 sampling points, indicating that most free floating green tide algae could not survive in Qingdao coast. In addition, the results showed that the chlorophyll content was in direct proportion to the photosynthetic activity of U. prolifera(F=25.106,R=0.782,P<0.01), which provides a basis to explore the coupling relationship and consummate the evaluation system in the future.