In order to investigate the occurrence and sources of floating Sargasssum horneri in the coastal waters of Gouqi Island. This study used the raft attachment and benthic S. horneri in Gouqi Island as the research object. Based on the growth, vesicle formation time, and biomass change rules of the attached S. horneri, the occurrence time, biomass, and floating path of the S. horneri were analyzed from the source. The findings indicate the following: (1) The formation time of floating S. horneri from raft attachment is 5–6 months earlier than that from benthic sources, and that approximately 99.12% of the biomass of floating S. horneri in the coastal waters of Gouqi Island likely originates from raft-attached S. horneri, whereas only 0.88% of the biomass originates from benthic S. horneri. (2) The number of floating S. horneri strains formed after the raft-attached S. horneri detached from the attachment base was approximately 22×10⁶ , with a biomass of about 141938 t. The biomass of floating S. horneri solely from raft attachment on Gouqi Island accounts for 23.36% of the total biomass of floating S. horneri in the East China Sea, whereas the benthic S. horneri comprises 0.21%. (3) Based on the biomass of attached S. horneri in various regions of China, the timing of floating S. horneri occurrences, tidal currents, and the influence of the Yangtze River environment on the floating pathways of S. horneri, two potential sources of floating S. horner in China were identified. The northern source is located near the Liaodong Peninsula and Shandong, whereas the southern source is situated south of the Yangtze River estuary. These findings not only provide a reference for analyzing the sources and occurrence potential of floating S. horneri in the coastal waters of Gouqi Island but also offer some basis for tracing the origins of the “Golden Tide”.