Abstract: Abstract: Heat stress is a common agrometeorological disaster during rice production in the middle and lower reaches of Yangtze River of China, and rice yield decreases due to the heat stress. Exogenous chemical solutions have been sprayed on rice leaves to improve the heat tolerance of rice and reduce the damage during the heat stress. This paper was aimed to evaluate the effects of 3 different exogenous chemical solutions on photosynthesis, chlorophyll fluorescence characteristics of flag leaves and rice yield under the heat stress condition. Pot experiments were conducted at Agricultural Meteorological Experimental Station of Nanjing University of Information Science and Technology (32.2(N, 118.7(E), in China in the year of 2016 and 2017. Early indica hybrid rice variety named Lingliangyou 268 (Oryza.sativa L.) was used in the experiments. High temperature treatments were conducted at late jointing stage (20th June, 2016) and late booting stage (5th July, 2017), respectively. Before the high temperature treatment, 3 different chemical solutions, including KH2PO4 (T1) solution with concentration of 22.04 mmol/L , CaCl2 solution (T2) of 20.0 mmol/L and Na2SiO3·9H2O solution (T3) of 2.5 mmol/L, were sprayed separately and evenly on all leaves for 3 days. For comparison, distilled water was sprayed as the control treatment (CK). After that, all treatments were put in climate chambers for 5 days to simulate a severe heat-stressed environment during the rice key stages, in which (40(0.5) °C from 6:00-18:00 was set as daytime temperature and (30(0.5) °C from 18:00 to 6:00 next day was set as nighttime temperature with an average of 35 ℃ in each day. In 2017, the photosynthetic light response curve and chlorophyll fluorescence of flag leaves of different treatments were measured at the 5th day of the heat-stress treatment and the 5th day after heat-stress respectively. The parameters related to the photosynthetic light response characteristics, such as the light saturation point (LSP), light compensation point (LCP), apparent quantum yield (AQY), dark respiration rate (Rd) and maximum net photosynthesis rate (Pmax) were calculated on the basis of observed data. And the chlorophyll fluorescence parameters, such as the maximum photochemical efficiency(Fv′/Fm′) of photosystem II (PSII) under light adaptation, actual photochemical efficiency (ΦPSⅡ) of PSII, photochemical quenching coefficient (qP) and non-photochemical quenching coefficient (NPQ) were also calculated. The final yields were observed too. The results showed that, compared with CK, all 3 chemical solutions were able to increase the photosynthetic activity, particularly to raise the LSP, Pmax, Fv′/Fm′, ΦPSⅡ, and qP of flag leaves, and the NPQ was reduced. Furthermore, based on the evaluation by the subordinate function, it indicated that all 3 chemical solutions demonstrated positive effects on the resistance of rice leaves to heat stresswith the effect order of T1>T2>T3. In the terms of yield, when compared with CK, the yields of T1, T2 and T3 increased by 42.67%, 29.70% and 20.01%, respectively. It was demonstrated that spraying KH2PO4, CaCl2 and Na2SiO3·9H2O chemical solutions were beneficial to promote photosynthetic activity of rice flag leaves and significantly increased the rice yield.