This paper briefly discusses the injured symptom of trees by acid rain and detailed reviews of the injured mechanism of acid rain to trees and ecosystems. The main reasons of acid raind amage to trees are as follows: Firstly ,H + from acid rain stimulates cell membrane which changes cell penetrability and results in the destruction of cell atructures. Secondly, it is the destruction of cell strutures that changes the existing state of enzynes, reduces the activate of enzymes, loads to the imbalance of metabolisn. Finally, cell disintegrates and tree dies. (The effects of acid rain on forest ecosystems include: Acid rain leach forest canopy and soil which lose nutrient elements in forest ecosystems; Soil acidify caused by acid rain reduces usable mutrients in soil which make AI and other metal toxic to tree, affects the activate of soil mierobiota, restrains the decomposition of litter and soil. )In the meantime, it is the tree growth decline caused by acid rain that has forest ecosystems invaded easily by dueases and insects. The author proposed that it is the essential countermeasures for preventing the effects of acid rain, on forest to reduce and eliminate pollution source, to develop integrated research and to take silvicultural mearsures etc.