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Furfurylation with a low concentration of furfuryl alcohol (FA) promotes the improvement of the properties and the effectiveness of FA on cell–wall action without darkening the furfurylated wood to the point that it affects its applications. In this paper, the effects of furfurylation on the hygroscopicity and water uptake dimensional stability of poplar (Populus sp.) and Chinese fir (Cunninghamia lanceolata) were analyzed. Meanwhile, the distribution of FA resin, the relationship between wood and water, the change in pore size distribution, and the weight percentage gain and cell wall bulking coefficient of wood were also investigated. The results were as follows: (1) A low concentration of FA could better enter the cell walls of the Chinese fir than the poplar, as FA resin was almost cured in the secondary walls, cell corners, and compound middle lamellae when a 10% concentration of FA was applied to the Chinese fir and poplar. When the FA concentration was increased to 30%, there were no significant increases in the amount of FA entering the cell walls and the amounts of FA cured in the cell lumen of the poplar were greater than those of the Chinese fir. Meanwhile, the modification of cell walls was more suitable in poplar than in Chinese fir. (2) The pointed ends of the pit chambers and the pit apertures (800–1000 nm) in the poplar and the small pores of the pit membranes and the pit apertures (1–6 μm) in the Chinese fir were partially deposited by the FA resin, which formed new pores in the size ranges of 80–600 nm and 15–100 nm, respectively. The porosity of the poplar was greater than that of the Chinese fir, and the bulk density of the poplar was less than that of the Chinese fir before and after modification. (3) Furfurylation with a low concentration of FA was able to better reduce the equilibrium moisture content, improve the anti-swelling efficiency, and enhance the dimensional stability of the poplar wood compared to the Chinese fir. Furfurylation effectively reduced water uptake due to the hydrophobic property of the FA resin. The water uptake of the Chinese fir increased by 17%–19% in second cyclic water soaking when treated with FA with various concentrations, which indicated the loss and leaching of FA resin during the test. Low-field NMR was used to demonstrate that the furfurylation not only reduced the amount of water but also affected the combination state of bound and free water with wood. Thus, furfurylation at a low concentration is a feasible method by which to extend applications of furfurylated wood.