The sea area near Gouqi Island harbors expansive floating colonies of Sargassum horneri, whose specific source remains unidentified. Despite this, the distribution of S. horneri in the intertidal zone is limited. Hence, analyzing the growth changes of both intertidal zone and raft S. horneri is essential for understanding the origins of “gold tide.” Previous studies have demonstrated a correlation between the growth of intertidal zone and S. horneri and the changes in effective accumulated temperature. Furthermore, it is suggested that temperature and irradiance may have varying effects on the growth of intertidal zone and raft S. horneri. In this study, we aimed to explore the specific sources of floating S. horneri in the sea area near Gouqi Island. We also aimed at elucidating the growth patterns of both intertidal zone and raft S horneri and identifying the required effective accumulated temperature. Additionally, we aimed to discern the distinct impacts exerted by temperature and irradiance on these growth patterns. Intertidal zone and raft S. horneri were collected monthly between September 2022 and June 2023. The temperature and irradiance changes during the life cycle in this period were recorded, and the variations in different plant traits, such as plant height, wet weight, longest primary branch length, were analyzed. Lastly, the effects of temperature and irradiance on the growth of S. horneri were also investigated. The results are summarized below: Average plant height, wet weight, and longest primary branch length of raft S. horneri were (168.1±21.0) cm, (147.6±24.1) g, and (131.8±26.4) cm, respectively, which were much larger than those of intertidal zone S. horneri [(67.9±22.5) cm, (62.4±16.7) g, and (46.4±10.6) cm]. Before the breeding period, the growth rates of wet weight, plant height, and longest primary branch length of raft S. horneri were much higher than those of intertidal zone S. horneri. Raft S. horneri grew rapidly compared to intertidal zone S. horneri. The low temperature condition of about 9 ℃ limited the growth of S. horneri, and the stable irradiance around 35 μmol/(m² ·s) may be the key factor that leads to the effective accumulated temperature required for the growth of raft S. horneri. This value was much smaller than that of intertidal S. horneri. Additionally, the breeding time of raft S. horneri was 1–2 months earlier than that of intertidal zone S. horneri, and the growth rate of the former was much greater than that of intertidal zone S. horneri. Raft S. horneri may contribute significantly to the floating S. horneri population in the sea area near Gouqi Island. Our results illustrate that the growth rate of plant height, wet weight and longest primary branch length of the raft S. horneri was much faster than that of the intertida zone S. horneri, and it suggest that irradiance is the key environmental factor affecting the growth and reproduction of intertidal zone and raft S. horneri. These results provide a scientific evidence for identifying the source of floating S. horneri, which may assist its efficient cultivation in the sea area near Gouqi Island. In addition, the study confirmed the interference of external factors; for example, the impact of waves on the growth of intertidal zone S. horneri was much greater than that on the growth of subtidal zone S. horneri. Hence, the maximum plant height of intertidal zone S. horneri at similar temperatures and greater irradiance was smaller than that of subtidal zone S. horneri, and the decay was faster than subtidal zone S. horneri.