Taking the Pinus sylvestris var. mongolica sand-fixing plantations at different development stages (24-, 29-, 39-, and 43 years old) in Nenjiang Sandy Land as test objects, this paper studied their population structure, understory species composition, and species diversity. No regenerated seedlings were found in all the four P. sylvestris var. mongolica plantations. The mean individual height and DBH of the populations differed significantly with development stage. With the increasing age of the plantations, the proportion of small-sized individuals decreased obviously, while that of large-sized individuals increased, population tended to mature, and the diameter structure except 43 years old plantation was in normal distribution. A total of 33 understory plant species were recorded, belonging to 28 genera and 15 families. Setaria viridis was the dominant species, but its dominance decreased gradually with increasing age of the plantations. With the increase of plantation age, the proportion of annual plants decreased, while that of perennial plants increased. The Simpson index and Pielou index had no significant differences among the different aged plantations, but the richness index, Shannon index, and Alatalo index of 39 years old plantation were significantly higher than those of 24 years old stands, suggesting that the species diversity of the community improved with time.