Abstract: The pine forest ecosystem we have studied is situated at the purple mountain not far from Dr. Sun yat-san’s Mausoleum, Nanjing, Jiangsu Province. Masson’s pine trees (Pinus Massoniana Lamb.) constitute more than 80% of the total composition of the pine stand which are 12-14 years old, with a canopy density of 0.7, stand density 120 trees/mu (or 1800 trees/ha.), stand height 5.5m, and average breast-height diameter 11 cm. The undergrowth is mainly intolerent grasses, 60-100 cm high, with a coverage of 40-50%. The soil is yellow brown earth, heavy loam to clay.A formula for the moisture equilibrium in forest soil has been suggested in this paper to study the variations of soil moisture and their effects on tree growth. The main conclusions we arrived at are as follows:The annual variations of soil moisture in a pine stand can be divided into four phases in Nanjing region. During the phase from April to June, when water supply is most important for tree growth, soil moisture there is fairly sufficient, though a little uncertain at times.There is no significant variation of soil moisture beneath 20 cm of the ground. Soil moist ire there is much more stable than that at the above layer. So if we adopt some measure to make the roots of trees penetrate into deeper layers of the soil, they will absorb more water and grow better.The height growth of trees and the growth of early wood and late wood are closely related to the hydro-thermal coefficient of climate and the available water. In the April-June phase, the multiple correlation coefficient of the average height growth of trees to the hydro-thermal coefficient of climate and the capacity of available water is 0.962, and that of early wood growth of trees to those is 0.947- In the July-September phase, such a multiple correlation coefficient of late wood growth to those is 0.851- With the analyses of the correlations it is possible to use the hydro-thermal coefficient, especially together with the available water supply in soil, to predict the growth of the stand.Pine stand can make the soil desiccative during the growing season and moist durinq the dormant season in the mountainous region in Nanjing.