In order to optimize the indoor culture conditions of growth and synchronous reproduction of Sargassum thunbergii, young seedlings with 5 cm length were used as materials to investigate the effects of different temperatures(15,20,25 ℃),illumination intensity［60,100,160,200 μmol/(m²·s)］and nutrient ratio［KNO₃(mg)∶KH₂PO₄(mg)=10∶1,5∶1,1∶5,1∶10］on the growth and sexual reproduction synchronization by measuring the growth index,observing the morphological changes as well as the formation of reproductive organs and reproductive characteristics.This investigation focused on the optimal combinative conditions of indoor growth,reproduction of seedlings,early development of receptacles and synchronization of fertilization.The results indicated that the gradation of different environment factors that influenced the length growth rate of S.thunbergii was temperature>nutrient ratio>illumination intensity,and the factors that influenced the growth rate of mass was temperature>illumination intensity>nutrient ratio.Low temperature could induce the rapid emergence of pneumathode.The formation of branchlets had more relation to nutrient ratio and in general,the higher proportion of KH₂PO₄ in nutrients resulted in later emergence of branchlets.Temperature effect was more significant for receptacles emergence time in our results and zygotes formation and release were also regulated by temperature conspicuously.The higher the culture temperature,the slower the growth rate of receptacles and the later the time of egg fertilization.The pigment content decreased with increasing the illumination intensity.Using culture conditions control for the alga thalli,indoor culture of this species could be done 3-4 months earlier than thalli matured in nature to complete its life cycle.For mature thalli of 1g wet,an average of about 300 seedlings could be collected from the substrate well.So it was possible that the generation of different positions and organs of S.thunbergii could be induced by controlling the culture conditions in laboratory to achieve early development of receptacles and synchronization of fertilization.The data could be used for theoretical and practical references for the technical route of large-scale cultivation of highquality S.thunbergii seedlings.