The relationship of foreign trade and forest degradation is an important research topic in trade and environment. Based on dynamic optimal control method, the interrelation of forest resources' stock dynamic and foreign trade, market power and foreign debt or asset, is exploited. At the same time, the condition and change law of forest degradation is analyzed. Pontryagin Maximum Principle demonstrate a character which if shadow social price of forest resource has a higher price than, demand of forest resource exceed the supply, forest resource decreases, hence lead to forest degradation, even extinction. Some conditions are satisfied, the stock of forest resource and trade terms change in the same direction, and the stock of forest resources and market power varied in same direction. Existing foreign debts, and the international capital market, stock of forest resources will not be dried up.