Abstract: The subsequent effect of low temperature on photosynthesis in pot cultured tobacco seedlings were studied with a CIRAS-1 photosynthetic system, and related photosynthesis indexes were tested.The results were as follows:1) after 72h treatment at 3℃, the maximum rate of RuBP regeneration, carboxylation efficiency, stomatal conductance and intercellular CO₂ concentration decreased significantly, apparent quantum yield declined slightly, and the reversible decline of net photosynthetic rate was observed;2) after 72h treatment at 8℃, no remarkable change was observed in the net photosynthetic rate.