Costaria costata is a potential economic kelp. Light intensity is one of the key factors that regulates the growth, photosynthesis, and metabolism of seaweeds, and is also the main factor affecting the morphology and vertical distribution of seaweeds. Therefore, it is important to study the tolerance of C. costata to light intensity for commercial farming. The growth, photosynthesis, nutrient composition, and antioxidant capacity of young sporophytes of C. costata were studied under different light intensities of 30–120 μmol/(m² ·s) under laboratory conditions to reveal the adaptability of C. costata sporophytes to light intensity and support the cultivation of this species in marine areas. The results showed that the young sporophytes grew fast at 30–120 μmol/(m² ·s), and both the relative growth rate (RGR) and F_v/F_m were the highest at 60 μmol/(m² ·s). The total photosynthetic rate and the maximum apparent photosynthetic rate increased with light intensity at the early stage (5 d), and reached the highest at 60 μmol/(m² ·s) at the late stage (10 d). The respiration rate (R_d) was significantly lower at 60 μmol/(m² ·s) than at the other light intensities. The contents of chlorophyll a, fucoxanthin, and carotenoid did not differ significantly among the varying light intensities. The content of crude protein, fat, and crude fiber was positively correlated with light intensity, while the content of carbohydrate was negatively correlated with light intensity. The results indicated that light intensity of 60 μmol/(m² ·s) was suitable for the growth of young sporophytes of C. costata. Furthermore, the content of soluble proteins increases under high light intensities while the activities of ASAFR, SOD, APX were higher under low light intensities. Moreover, the alga had the same response mechanism, namely enhanced respiration, under either low light or high light stress.