This paper studied the dynamics of reproductive allocation (RA) of Sargassum thunbergii during its sexual reproductive season and the related environmental factors at the Taiping Cape of Yellow Sea. The sexual reproduction of S. thunbergii initiated in early June, peaked in mid July when the sea water temperature was about 22 degrees C (the mean proportion of biomass allocated to reproductive organs on July 19 was 76.7%), and ended in late August. The RA had a significant linear correlation with the average length of thallus branches (r = 0.855, P wave strength > tidal level x wave strength.