Abstract: The osmoregulation matters were investigated in Casuarina equisetifolia seedlings under acid rain stress with a series of acidity (pH2.5～4.5)for 3 months.The results show that the soluble sugar contents in seedling twigs under acid rain stess with low acidity are lower than that of control,however,it is significantly higher under moderate and intensive acid rain stress.The soluble protein contents of the twigs increase under the treatment of acid rain,and the proline and GSH contents increase as well. The high concentration of osmoregulation matters in Casuarina equisetifolia seedlings under acid rain stress is accumulated to improve the physiological responses of osmoregulatants.