[Objective] In order to provide reference for understory herbaceous communities management in Pinus sylvestris var. mongolica plantations, the dynamic of understory herbaceous communities were investigated and the relationships between diversity index and stand age in P. sylvestris var. mongolica plantations were explored. [Methods] The P. sylvestris var. mongolica plantations with different ages (8, 18, 27, 35 and 48-year-old) in Zhanggutai area in the Southern Horqin sandy land were selected as the study objects, the vegetation species, quantity, coverage and biomass of these plantations were investigated and the effects of stand age on diversity index were quantified. [Results] The importance value of Digitaria sanguinalis was the highest in all ages of the P. sylvestris var. mongolica plantations, indicating that the D. sanguinalis could adapt well to understory environment and was the dominant species of understory vegetation. With the increasing of stand age, the biomass and diversity of understory herbaceous vegetation decreased initially and then increased, and the lowest values occurred in the 27-year-old P. sylvestris var. mongolica plantations. Stand age showed binary regression relationships with Patrick richness index, Shannon-Wiener index, Pielou evenness index and Simpson dominance index. [Conclusion] The degradation phenomena existed understory herbaceous vegetation in half-mature and near-mature P. sylvestris var. mongolica plantations, thus artificial promoting measures such as thinning, intertillage and enclosuring should be adopted to promote herbaceous vegetation growth and succession.