The causes of natural wetland degradation were analyzed using the theory of exterior and property right combining some economical characterises.The wetlands were over explored mostly because the wetland is used as a resource of common property equally shared by everyone.As exterior resources, no enough compensation were given to the positive exterior of wetland, and no enough punishments were paid for the negative exterior.The characters of the wetland resource directly make the property right of the wetland divided, the owner can't entirely get the benefits from the wetlands, neither can control the profits, which led no investments on the wetlands.Meanwhile, the markets can't reflect the real values of the wetlands resources for the failure of supply and demand of markets, the failure of markets regulation, the lack of the price of wetland, and the low price of wetland.These factors had caused the wetland functions lost, the areas decrease and etc.