Abstract: To study the effects of exogenous silicon on tobacco photosynthesis under cadmium stress, hydroponic experiments were conducted on Yunyan 87 with exogenous silicon in the form of potassium silicate (K₂SiO₃) and cadmium in the form of cadmium chloride (CdCl₂·2.5H₂O). KNO₃ and HNO₃ were used to control the effects of increased pH and K+ of nutrient solution caused by potassium silicate, five treatments were set as follow: control, 0.05 mmol·L^-1 Cd²⁺(Cd_0.05), 0.10 mmol·L^-1 Cd²⁺(Cd_0.10), 0.05 mmol·L^-1 Cd²⁺+1.00 mmol·L^-1 SiO₂(Cd_0.05+Si), and 0.10 mmol·L^-1 Cd₂₊+1.00 mmol·L^-1 SiO₂(Cd_0.10+Si). The results showed that comparing with the control, Cd_0.05 and Cd_0.10 treatments significantly decreased the length, leaf area and biomass of tobacco plants, net photosynthetic rate was also significantly reduced by 52.02% and 73.75%, respectively. Exogenous silicon could alleviate the harmfulness of cadmium on the tobacco. Comparing with Cd_0.05 and Cd_0.10 treatments, Cd_0.05+Si and Cd_0.10+Si treatments increased the stomatal conductance of tobacco leaves by 259.90% and 13.29%; raised chlorophyll content by 43.47% and 92.33%; and promoted net photosynthetic rate by 99.90% and 72.78%, respectively. Exogenous silicon could promote the apparent quantum efficiency and carboxylation efficiency of tobacco under cadmium stress. Correlation analysis indicated that net photosynthetic rate significantly negatively correlated to the cadmium content in chloroplasts of tobacco, while silicon could reduce the cadmium content in chloroplasts, which was one of the underlying mechanisms that silicon increased the photosynthesis of tobacco under cadmium stress.