To develop a humectant with excellent moisturizing properties, this study used Korean pine needles as raw materials to prepare pine needle polysaccharides, which were then carboxymethylated. The structural characteristics and moisturizing behavior of the polysaccharides were investigated. By comparing the physicochemical properties of the polysaccharides before and after carboxymethylation, the effects of the modification on the molecular structure and hygroscopic properties were analyzed, and kinetic model fitting was performed for the hygroscopic process. Furthermore, carboxymethylated pine needle polysaccharides were compounded with potassium lactate at different ratios. The moisturizing performance of the compound system was evaluated by the equilibrium moisture content, and its wetting characteristics and water distribution state were analyzed through contact angle testing and low-field nuclear magnetic resonance (LF-NMR) technology. The results showed that under the conditions of an extraction time of 2 h, an extraction temperature of 80 ℃, a liquid-to-solid ratio of 80∶1 (mL/g), and an ultrasonic power of 250 W, the yield of pine needle polysaccharides was 5.48%. After carboxymethyl modification, the molecular weight of the polysaccharides decreased while the monosaccharide composition remained basically unchanged, and the maximum equilibrium moisture content in the filter paper model could reach 6.03%. When the mass ratio of carboxymethylated pine needle polysaccharides to potassium lactate was 1∶3, the equilibrium moisture content of the compound system further increased to 6.25%. The results of contact angle and LF-NMR analyses indicated that the compound humectant had good wetting properties and could significantly increase the proportion of bound water in the system. The carboxymethylated pine needle polysaccharide–potassium lactate compound system exhibited excellent moisturizing performance and showed potential for application as a composite humectant.