Based on stem analysis and sap flow measurement, this study was conducted in subtropical China to clarify the primary productivity and the contribution of afforestation to carbon cycling, as well as the coupling relationship between carbon sequestration and water use. The growth model for Pinus elliottii was established, including volume table, biomass equation, diameter increment etc. Sap flow was measured for interpreting the relationship of primary productivity with water use. The diameter increment declined even from the beginning of afforestation, which implies the inadaptability of the exotic species in China. The stand density was reduced from the initial 5 000 stems/hm2 to the present 1 599 stems/hm2 in 20 years by self thinning. The stand stock was 157 m3/hm2 with its maximum of 209 m3/hm2, and the volume increment was 22 m3/(hm2 a). The above-ground biomass was 72.061 t/hm2, and its growth rate was 8 493 kg/(hm2 a) or 4 370 kgC/(hm2 a), which is approximately consistent with the result (NEE) by means of eddy covariance. The sap flow velocity was 21 495 cm3/(cm2 a), and the total transpiration through canopy layer was 4 769.643 t/(hm2 a) or 477 mm in terms of precipitation, which accounted for 48% of the annual rainfall. The maximum sap flow per year was about 634 mm. The ratio between water consumption and dry matter accumulation was 562∶1. There was a significantly positive correlation between NPP and sap flow velocity, in both daily value and monthly average.