Abstract: China’s largest Sargassum fusiforme cultivation area is located in Dongtou District, Wenzhou City, Zhejiang Province. Seedling propagation using fertilized zygotes has become the main source of cultivated seedlings. Nevertheless, local production is constrained by multiple adverse factors including seedling genetic mixing, long nursery cycle, extreme weather, and fish predation, leading to unstable seedling supply. In this study, fertilized zygotes of local S. fusiforme were used as experimental materials to conduct seedling breeding tests and to establish an indoor unattached suspension culture system. We systematically investigated the independent and interactive effects of key environmental factors (temperature, salinity, and light intensity) on the development of zygotes, juvenile sporophytes, and seedlings. The results showed that zygotes developed normally at 15—25℃ and salinity of 20‰—30‰. The optimal culture conditions for juvenile sporophytes were 20℃, salinity of 25‰—30‰, light intensity of 70 μmol photons/(m²·s) and seedling density ≤5 ind./mL. Juvenile sporophytes with 2—3 leaves could be obtained after 30 days of culture. For seedling cultivation stage, the optimal parameters were 20℃, light intensity of 100 μmol photons/(m²·s) and seedling density of 5—20 ind./L. Seedlings cultivated via indoor suspension culture reached the transplant standard within only 3 months, which was 2 months shorter than conventional open-sea seedling raising. Theseseedlings featured a compact morphology with intertwined rhizoids. After 90 days field cultivation trial, the individual wet weight of seedlings from the indoor suspension treatment increased by 30.76% compared with the control group. This study demonstrates that the established indoor suspension culture system effectively reduce the industry’s reliance on wild natural germplasm and northern introduced seedlings, mitigates high-temperature summer stress, and enables localized, controllable, and standardized seedling production. The findings provide theoretical references and technical support for large-scale seedling breeding, germplasm conservation, and strain selective in S. fusiforme.