Abstract: The effects of stimulated acid rain on the fruit-dropping and fruit quality of 8-year old longan are studied in this paper．Thc results show that the fruit dropping percentage increases significantly，while the percentage of TSS，the ratio of TSS to total acid and that of simuluble sugar to acid in the fruits reduce remarkably under the stress of simulated rain with pH≤ 3．5．The fruit appearance is damaged and the weight per fruit and edibility percentage decrease obviously in the treatment of acid with pH2.5.