Sexual reproduction characters of Sargassum cinereum were observed by tissue culture in vitro. Effects of light intensity 20, 40, 60, 80, 120, 160 μmol/(m²·s) on sexual reproduction and early development of S. cinereum germlings were studied. The results showed that four to six eggs were released from each female conceptacle. Freshly released unfertilized eggs had single nucleus, which then changed into two to eight nuclei by nuclear division. The first two cell divisions after fertilization were transverse, and the following two cell divisions were all longitudinal. Through subsequent countless cell divisions, a pyriform germling was formed. Light intensity insignificantly influenced the size of eggs, the size and the cell division velocity of zygotes. However, variations in rhizoid number, rhizoid length and thalli growth were highly significant under different irradiances. The first branches on germlings were observed after being cultured for 8 days. Suitable growth of i>S. cinereum occurred under irradiances from 40 μmol/(m²·s) to 120 μmol/(m²·s). Optimal growth of these germlings occurred under irradiances from 40 μmol/(m²·s) to 60 μmol/(m²·s) in early development stage and from 80 μmol/(m²·s) to 120 μmol/(m²·s) after one leaflet was formed, respectively. These results may provide theoretical basis for artificial seedling culture of i>S. cinereum.