Sustainable development is a hot topic in the world. And how to judge sustainable development is a difficulty in the topic. Applying environmental economics, the article gives sustainable development's four characters: the system's total output should be larger than total cost; it should increase by degrees; it should be coordinated; it should be stable. In theory, they can define sustainable development quantitatively, In practice, they are used to judge the development situation of Dafeng, Jiangsu Province, and helped the city to draw the sustainable development policy and evaluate the policy's effectiveness.