The abnormal increase of drifting brown alga Sargassum horneri was initially documented in 2007. It formed blooms along the coast of East China Sea and Yellow Sea in 2017. In this study, we investigated the changes of specific growth rate and resource accumulation of drifting S. horneri in response to temperature and nitrogen richness at different growth stages under laboratory condition. The investigation lasted from June 2015 to April 2016 with the observation made every two months. The results showed that the life cycle consists of a few growth stages dividable with morphological characteristics. The growth can be divided into shedding and withering (August), rapid growing (October to September), slow growing (February), rapid growing (April) and maturation (June) stages. Under the experimental condition, algal segments were found to grow at temperatures ranging from 5 to 25°C in 12 days even when nitrogen is deficient. A significant difference in the special growth rate (SGR) between nitrogen-enriched and nitrogen-removed treatments was found in most months ( P < 0.05). SGR was lower in August and February than that in other months. Nitrogen and chlorophyll contents in algal segments were different among different temperatures, nitrogen supply and seasons. Nitrogen content was higher in February and April than that in other months in both nitrogen-enriched and nitrogen-removed treatments. The results showed that the demand of S. horneri for nitrogen increased in spring when it grows fast. It is likely that the high temperature and nitrogen concentration in winter and spring lead to the high biomass accumulation of drifting S. horneri.