Effects of different temperatures (5-34 ℃),different salinities(10-50),different nutrient concentrations and different nitrogen forms on chlorophyll fluorescence parameters of young seedlings of Sargassum thunbergii were researched in this paper.The main parameters determined in the study included the optimal chlorophyll fluorescence quantum yield(F_v/F_m)and the maximum relative electron transport rate(rETR_max)of photosystem II.The results are shown as follows:(1) If nitrogen concentration or phosphorus concentration was higher than 8 mg/L and 1.2 mg/L respectively,or temperature was greater than 28 ℃,there were significant effects on the photosynthesis of young seedlings of S.thunbergii.(2) Short time treatments in 5-15 ℃ and six hours treatments in 10-50 salinity of seawater did not affect the F_v/F_m of young seedlings of S.thunbergii obviously.(3) If nitrogen concentration and phosphorus concentration was 2-4 mg/L and 0.2-0.8 mg/L respectively,and moreover,NH⁺₄-N∶NO^-₃-N was equal to 1-3,it was beneficial to the photosynthesis of young seedlings of S.thunbergii.