The regulation of different calcium forms, namely CaCl₂, Nano-calcium and Manntiol-calcuim, on the gas exchange and fluorescence of tomato leaves under heat stress was investigated. The results showed that all forms of calcium  alleviated the decrease of chlorophyll a and carotenoid contents in leaves of tomato seedlings under heat stress, enhanced the net photosynthesis rate (P_n), transpiration rate (T_r) and stomatal conductance (g_s) to varying degrees, reduced the quantum yield of non-regulated energy dissipation $$Y(NO)$$ of PSII and quantum yield of non-photochemical energy dissipation in PSI due to acceptor side limitation $$Y(NA)$$ , promoted the regulated energy dissipation $$Y(NPQ)$$ and quantum yield of non-photochemical energy dissipation in PSI due to donor side limitation $$Y(ND)$$ , and increased the calcium content in leaves. Generally, manntiol-calcium and nano-calcium were more effective than CaCl₂, and more suitable to enhance the photosynthesis of leaves oftomato seedlings under heat stress.