Young sporophytes of Sargassum fusiforme were cultured at decreased CO2 (20 μatm), ambient CO2 (400 μatm), and high CO2 (1000 μatm), and then the quantum efficiency of open photosystem II (Fv′/Fm′), initial slope of the rapid light curves (α), and relative maximum photosynthetic electron transport rate (rETRm) of the algae under different temperatures and light levels were measured. The study aimed to investigate how the decreased CO2 and high CO2 supply affected the growth and photosynthetic functions of S. fusiforme young sporophytes. While both lowered and increased CO2 supply significantly reduced the growth rates of the alga, greater declines were observed under decreased CO2. The Fv′/Fm′, α, and rETRm of alga remained stable after short-term (120 min) exposures to 18, 22, and 26 °C, as well as to highlight (300 μmol photons m−2 s−1), with no significant difference among the three CO2 supply treatments. Hence, neither decreased nor increased CO2 affected the photosynthetic responses of S. fusiforme young sporophytes to temperature and high light. However, the Fv′/Fm′ of the three CO2 treatments declined by 72% under 60 μmol photons m−2 s−1, suggesting its sensitivity to short-term low light. These observations are crucial for the improved management of S. fusiforme for commercial farming, while ensuring its sustainable production and supply amid seawater pH shifts brought about by global climate change.