Injury mechanism of acid rain on the photosynthetic apparatus of plants has been one of the hot spots of the ecological research. To elucidate the relationship between the damage of reaction center and the accumulation of free radicals in photosynthetic system due to foliar acidification, one-year old longan (Dimocarpus longana Lour. cv. Wulongling) seedlings grown in pots were used to study the effects of simulated acid rain stress on chlorophyll fluorescence parameters, free radical metabolism and their recovery. The results showed that, under acid rain stress, the curve of chlorophyll fluorescence was quickly changed and PSⅡ reaction center was damaged. Maximum photochemistry efficiency of PSⅡ(F_v/F_m), the activity of PSⅡ reaction center (1/F_O-1/F_M) and the density of active reaction center (RC/CS_O) decreased significantly after 5d stress with acid rain of pH2.5. The closure degree of active reaction center (V_J) and the percentage of inactive reaction center (Non-Q_A and Non-Q_B) increased, whereas the Q_A was reduced rapidly. The oxygen releasing complexes were destructed, and the number of electron transducer of PSⅡ receptor (S_m), the electronic conversion efficiency (ψ_O), the transduction rate of electron (φ_Eo) all decreased significantly. These results displayed that foliar acidification inhibited the linear electronic transduction of PSⅡ reaction center. The results of free radical metabolism of longan indicated the contents of O₂^·－, H₂O₂ and MDA increased significantly after 5d stress with acid rain of pH2.5. AsA and GSH converted to oxidized form, the contents of reduced form decreased, and the activities of SOD, APX, MDAR, DHAR and GR all reduced significantly. The inability of scavenging the free radicals in chloroplast in time resulted in the attack of photosynthetic organs by the redundant free radicals and thus the damage of PSⅡof longan leaves. Taken together, we concluded that acid rain stress damaged the oxidation and reduction sides of the reaction centers of PSⅡof the leaves in longan, and further decreased the linear electronic transduction to result in the shortage of assimilatory power which weakened the scavenging of free radicals, the accumulation of free radicals in chloroplasts destructed photosynthetic apparatus more.