Abstract: .The maximum photosynthetic rate of litchi saplings leaves grown under a CO₂ partial pressure of 77?5 Pa was 23% lower than that of 39.3 Pa, and slight decreases in respiration rate in light and CO₂ compensation point excluding of respiration in light were observed. The maximum rates of carboxylation (V_cmax) and photosynthetic electron transport (J_max) were decreased in saplings grown under 77?5 Pa CO₂ partial pressure. It may suggest that there was a lower energy level of photosystem I(PSI) in the saplings leaves under 77?5 Pa CO₂ partial pressure, and free radical yield decreased by 39% in the saplings under 77?5 Pa CO₂, as comparied with that under atmospheric CO₂. The percentage of infection by Peronophythora litchi increased from 1.8% under atmospheric CO₂ to 9.5% under enriched CO₂. It may mean that the decreasing photosynthetic and respiration metabolism would make it lower the production of O₂^-., and leaves were infected by P.litchi more easily. The controlling of the popularization of P.litchi for litchi plantation must be paid wide attention,as air CO₂ partial pressure will increase continuously.