Abstract: Ulva prolifera seedlings, which are the main component of the natural "seed bank" of green tide algae, play an important role in the occurrence and development of green tide. In order to understand the causes of the early outbreak of the green tide of U. prolifera and provide basic data for its early warning and prevention, we studied the growth and physiological responses of seedlings after the seedlings being cultured at two different CO₂ levels (LC: 400 μatm; HC: 1 000 μatm) in combination with three different photoperiods (LL: 10 L∶14 D; ML: 12 L∶12 D; HL: 14 L∶10 D). The relative growth rate of seedlings were significantly enhanced by elevated CO₂ under three light-dark regimes (P<0.05). The seedlings showed an obvious higher growth rate and a lower dark respiration rate (R_d) by HL treatment than by ML treatment. The impact of elevated CO₂ and illumination time on the photochemical performance was not obvious. Elevated CO₂ and longer illumination time had negative effects on chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoids (Car) content. The results suggest that the growth and physiological of seedlings are significantly influenced by elevated CO₂ and photoperiod (P<0.05). HC and HL promoted the growth of its seedlings and increased the possibility of outbreak of green tide caused by U. prolifera. This study provides basic data for indepth understanding of the causes of green tide algae outbreak.