In order to get suitable cultivating conditions, the effects of environmental factors on growth of the 60-days-old Sargassum horneri seedlings were investigated in laboratory. The results showed that, S. horneri seedlings survived at 15 to 30 ℃, and suitable temperature range was from 20 to 25 ℃. The seedlings grew up under light intensity from 500 to 7 500 LX, and the better light intensity range was from 2 500 to 4 500 LX. Seedlings could grow in the salinity range from 10 to 40, and the suitable salinity was 30. Long photoperiod (18L:6D) was more beneficial to growth of S. horneri seedlings. The low cultivation density (5-25·L^-1) was beneficial to growth and development of the seedlings. These results provided basic technical data for artificial seeding of S. horneri to spend summer indoors.