The non-structural carbohydrate (NSC), carbon (C), nitrogen (N) and phosphorus (P) concentrations in plant leaves can reflect the response and adaptation of plants and ecosystems to water-deficit environments. In this study, we analysed the relationship between the NSC, C, N and P in 25a Pinus massoniana needles through in situ observation during a 3-year continuous 100% rainfall exclusion experiment in Changting County, Fujian Province. The results showed that (1) there was no seasonal variation in needle NSC concentrations of the drought group, due to continuous rainfall exclusion; NSC concentrations increased significantly in the early stage and decreased in the late stage; soluble sugar concentration, the ratio of soluble sugar to starch in the drought group increased significantly in the later stage. (2) Compared with the control group, both N and P concentrations as well as the N:P ratio of P. massoniana needles in the drought group showed no significant differences in the early stage and significantly higher values in the later stage. (3) The needle NSC concentrations, soluble sugar concentrations, P concentrations and N:P ratio of P. massoniana correlated significantly or highly significantly with soil moisture content, and all of them were significantly or highly significantly correlated with P concentrations; however, there was no correlation with N concentration. Our findings indicate that P. massoniana could adapt to a continuous reduction in soil water content by adjusting the concentration and distribution of NSC and increasing the N and P concentrations in the needles. Additionally, the P concentration plays a facilitating role in the fluctuation of NSC concentration and the interconversion of soluble sugar and starch.