Abstract: In this study, using the method of replacing time with space, 10 years old, 24 years old, 34 years old, 43 years old and 54 years old Pinus koraiensis plantations were selected as the research objects. In order to explore the diversity characteristics of understory shrub and grass of P. koraiensis plantations of different ages in the mountainous area of eastern Liaoning Province, the changes of understory vegetation composition, dominant species and species diversity of 5 stands were recorded and analyzed through field plot investigation. The findings indicated that species composition of P. koraiensis plantation was abundant, and there are 34, 42, 54, 46 and 56 understory species at different increasing forest ages. From the perspective of species number, with the increase of stand age of P. koraiensis plantation, the total number of species and the number of herbaceous plant species showed an overall trend of increase, while the number of shrub plant species showed a trend of increase first and then decrease. From the perspective of important value, Euonymus alatus and Aralia elata var. glabrescens were used as dominant species in the shrub layer, Chelidonium majus, Phryma leptostachya subsp. asiatica and Cardamine leucantha are dominant species in the shrub layer. From the perspective of species diversity, in the shrub layer, S showed a fluctuating upward trend with the increase of stand age, and D and H showed a trend of increase first and then decrease, while J_D and J_H had little change between different forest ages. In the herb layer, the 5 diversity indices of S, D, H, J_D and J_H generally showed a continuous upward trend with the increase of stand age. The total number of understory species, the number of herbaceous plants and the index of herbaceous diversity of P. koraiensis plantation in the peak fruiting period were high. The study provides a reference for improving the diversity of understory vegetation, playing ecological benefits, maintaining and promoting the balance and stability of P. koraiensis plantation ecosystem.