Abstract: Expressing stand density quantitatively and determining its reasonable density are key scientific issues in forest management. Based on the forest mean crown width of trees, which directly reflects forest area of photosynthesis, and mean tree distance, which reflects the possibility of forest maximum spatial utility , both are influenced by stand density, a new index K, which is equal to the ratio of mean distance and crown width is proposed to digitally express the stand crowding degree. The value of K can directly and clearly reflect the crowding degree of trees, and the smaller the value is,the more crowded the trees are. It can also effectively reflect the congestion degree and its dynamic feature in the stand growing process. The reasonable interval of K value ranges from 0.9 to 1.1, and it is proven by the long-term planting density experiment data. The index K, which has the advantage of simple and easy to operate, is a more direct and scientific expression of stand density. It can be used to evaluate whether the forest needs management, and determine the right time for the first thinning and its management intensity. Eventually, it provides the basis for density control and adjustment.