Acid rain is a global environmental problem that has become a very serious issue in China coupled with the rapid economic growth. The loss of nutrients in forest ecosystems, especially calcium, as shown in a lot of references done over long-term observation and research, is one of the key factors which indicate that acid rain caused the decline of forest ecosystems. Calcium has been markedly leached from the soils by acid rain, mostly in acidic soils such as red soils. Acid rain has also leached calcium from the canopy of forest resulting in a decrease of the folia calcium content sometimes, imbalance of calcium homeostasis at the cellular level, impair of physiological responses. From the ecosystem aspect, acid rain changed the characteristics of calcium biogeochemical cycle, brought calcium to be depleted from the ecosystem in the surface runoff, reduced the bioavailable calcium pool and increased the turnover rate. Because acid rain influenced calcium at different levels of the ecosystem, it is important that we focus on studying the calcium depletion and responses of plants under long-term acid rain, which can help us in the progressive prevention and treatment of acid rain and the conservation of forest ecosystems.