Sargassum thunbergii is an economically important brown alga that is used as a perferred food for sea cucumber in China. However, reports on the reproductive biology of S. thunbergii are limited. This study observed the characteristics of mature receptacles. The effects of different temperatures, light intensities, and photoperiods on the egg release of mature S. thunbergii receptacles were investigated. A liquid-phase oxygen electrode system was used to obtain light saturation and light compensation points of egg and young thalli of S. thunbergii. Results showed that temperature was the key factor for the egg release of mature receptacles. The conditions most conducive to egg release were 20°C to 23°C temperature and 50 μmol photons m^-2·s^-1 to 200 μmol photons m^-2·s^-1 light intensity. In addition, the light saturation and compensation points of unfertilized eggs or young thalli at 2 d or 3 d postfertilization ranged from 90 μmol photons m^-2·s^-1 to 120 μmol photons m^-2·s^-1 and 14 μmol photons m^-2·s^-1 to 22 μmol photons m^-2·s^-1, respectively. The combined results provide a reference for the sporeling culture of S. thunbergii.