Abstract: Sagassum vachellianum, one of our country unique algae, has attracted great attention. This paper focused on the main reproduction season, occurrence of receptacles, mature and release of ovum, growth of rhizoids and early development of juvenile sporophyte of S. vachellianum from Gouqi Island. It was found that the breeding season of S. vachellianum was mainly from April to June, and the optimum water temperature of reproduction was ranged from 18℃ to 21℃. At the early stages, there were no significant difference between male and female receptacles, but the significant difference of shape appeared in later stages. The ovum development of S. vachellianum belongs to 8-nucle-I type, and the fertilization process occurred on the surface of female receptacles, and then the zygote began disengaged from receptacle after fertilization. In the prophase of juvenile sporophyte, rhizoid growth rate was faster than that of juvenile sporophyte. The first and second leaf developed and then differentiated into stem and branches. The study revealed the growth and development of S. vachellianum, to provide better reference for the artificial breeding of S. vachellianum.