Abstract: The protective effect of calcium on muskmelon seedling harmed by acid rain was studied. When muskmelon seedlings were sprayed with 30 mmol/L CaCl2 two times (once every 24h) ,the damage to muskmelon seedlings by acid rain at pH2.5 were reduced. The protective effect was because that CaCl2 raised photosynthetic rate, chlorophyll content and Ca2+ content, increased catalase activity, decreased malondialdehyde content, reduced cell membrane permeability and kept stability of cell juice pH value of muskmelon seedlings, etc.