Abstract: To investigate the physiological response characteristics of tomato (Lycopersicon esculentum) to silicon, the effects of silicon(Si) on the growth, H2O and CO2 exchange parameters of tomato were studied under the hydroponics with the 0 (CK), 0.6 (T1), 1.2 (T2) and 1.8 (T3) mmol/L silicon levels. The results indicate that silicon contents of tomato organs are increased dramatically with the increase of Si concentration in culture solution, and the differences in leaves are most significant. The SiO2 contents of leaves of T1, T2 and T3 are increased by 250.90%, 403.59% and 552.69% compared with the CK. Compared with CK, the plant growth and chlorophyll contents of T2 and T1 are higher, while T3 is no significant difference. Simultaneously, net photosynthetic rates (Pn) of T1 and T2 are also significantly higher than that of the CK, which are increased by 15.36% and 23.12%, and at 11: 00 oclock, Pn of all treatments reach their peaks, while Pn of T3 is 5.74% lower than that of the CK. Transpiration rate (Tr) of tomato leaves is decreased as the silicon concentration increased, and Tr of T1, T2 and T3 are reduced by 7.42%, 11.47% and 23.08% at 13: 00 oclock in contrast to CK. The instantaneous water use efficiency (WUEi) of tomato dealing with silicon is significantly higher than that of the CK, at 11: 00 oclock, WUEi of all treatments reach their peaks, and WUEi of T1, T2 and T3 are 22.22%, 35.47% and 17.52% higher than the CK. The experiment demonstrates that the appropriate silicon concentration for optimizing the tomato growth is 1.2 mmol/L.