Abstract: As the causative species of green tide outbreaks in the Yellow Sea, Ulva prolifera shows strong environmental adaptations at the seedling stage, which is crucial for the occurrence of green tides. In this study, we investigated the coupled effects of diurnal temperature variation (Constant temperature T1: 22  ℃; diel temperature fluctuation T2: 22  ℃ during the day and 18  ℃ at night) and different concentrations of dissolved inorganic phosphorus (DIP) (Low phosphorus LP: 0.24 μmol·L⁻¹; medium phosphorus MP: 1.2 μmol·L⁻¹; high phosphorus HP: 6.0 μmol·L⁻¹) on the growth and photosynthetic physiology of U. prolifera seedlings through an outdoor controlled experiment. The results show that, compared with the constant temperature treatment (T1), the diurnal temperature variation treatment (T2) significantly increased the relative growth rate (RGR), net photosynthetic rate (P_n), maximum relative electron transport rate (rETR_max), as well as the contents of chlorophyll a (Chl a) and carotenoids (Car) in the U. prolifera seedlings. High DIP concentration (HP) further promoted the increase in these physiological parameters, with these indicators reaching their maximum values under the combined condition of diurnal temperature variation and high DIP concentration (T2HP). Two-way analysis of variance reveals significant interactive effects between diurnal temperature variation and DIP concentration on RGR, Chl a, light use efficiency (α), soluble protein (SP) and soluble carbohydrates (SC) contents. The results indicate that the combination of diurnal temperature variation and high DIP concentration synergistically enhances the photosynthetic capacity and carbon assimilation efficiency of U. prolifera seedlings, providing a physiological basis for their explosive growth. The findings offer theoretical insights for understanding the environmental drivers behind early outbreaks of green tides in the Yellow Sea and for developing prevention and control strategies.