Abstract: To probe the effect of CO₂ enrichment on crop growth in greenhouse under different temperature and Photosynthetic Photon Flux Density(PPFD), infrared ray CO₂ gas analyzing instrument measuring method was adopted and the effect of CO₂ enrichment on lettuce photosynthesis was studied systematically. The comprehensive model of the effect on photosynthesis by CO₂ enrichment was established respectively at low and middle-high temperature. The results showed that within a certain range with the enhancement of temperature and radiation flux density, the promotion of CO₂ to photosynthesis improves, and adverse effect will appear above saturation point. Based on the result, the optimum combination of ecological factors is illumination 897.3 μmol·m^-2·s^-1. Temperature 28.9℃, CO₂ density 2 160 μL/L, and net photosynthesis rate Pn(CO₂) is 36.0 μmol·m^-2·s^-1.