Abstract: Seagrass meadow, an important coastal “blue carbon” ecosystem, is declining due to high input of organic matter, which could be effectively improved by addition of exogenous iron (Fe). In this study, different amounts of Fe (II) (0.00, 0.05, 0.15 and 0.50 g/kg) were added into two types(calcareous and siliceous) of sediments in greenhouse to examine the alleviating effects of Fe application on photosynthetic pigment (Chl a、Chl b、Cx), chlorophyll fluorescence parameters(F_m、F_v/F_m、F_v/F₀、Φ_PSII、ETR) and antioxidant enzymes (POD、SOD、CAT) of Thalassia hemprichii leaves, with starch (5 g/kg) added to simulate a high organic loading in seagrass meadow. The results showed that Fe addition in two types of sediments could both effectively improve the contents of photosynthetic pigments, the level of chlorophyll fluorescence parameters and the activity of antioxidant enzymes in a 30 day experiment and it was found that 0.50 g/kg was the most effective among all treatments. Furthermore, Fe addition had a very significant effect (P<0.01) on the contents of Fe²⁺ and S^2- in the interstitial water of sediments, the content of photosynthetic pigment, chlorophyll fluorescence parameters and antioxidant enzyme activity of Thalassia hemprichii leaves, while the sediment type had a very significant effect on Fe²⁺ and antioxidant enzyme activity (P<0.01). Additionally, the interaction between sediment type and Fe addition had a significant (P<0.05) or extremely significant (P<0.05) effect on photosynthetic pigment contents and antioxidant enzyme activities. In general, the effects of Fe addition on photosynthesis and antioxidant enzyme activities were higher than those of sediment type. This study revealed that exogenous Fe could effectively improve several physiological and ecological indexes of Thalassia hemprichiigrowing in alcareous and siliceous sediments, thus it can provide data and technical support for the protection and restoration of tropical seagrass meadow.