In this paper the relationships between forest density and index of decline disease, and soil moisture content were studied and determined in Pinus taeda plantations in Anhui province. The results showed that there are a closely correlation between them. The forest density is positively correlated with index of the disease, and negatively correlated with soil moisture content on a given tree age and site type. Thinning, therefore ,could reduce the index of disease and increase the growth of Pinus taeda.So scientific thinning should be used as a control measure of the disease in forest management.