Understory plant species composition and diversity were researched at different forest age of Pinus massoniana plantation in the hilly area of the upper reaches of the Yangtze River by typical sampling method. The results showed that plant species composition, diversity index and evenness index had an obvious volatility with increasing stand age, the total number of understory species was minimum at 5 years while maximum at 14 years; Rubus pirifolius in the shrub layer had a large advantage in different stage, Quercus serrata and Mallotus japonicus were dominant in middle-aged forest (14a), Rubus pirifolius and Melastoma normale were superior in mature forest (37a、39a). Miscanthus sinensis and Dicranopteris dichotoma in the herbaceous layer were the dominant species at different stage of plantation. Both simpson index and the shannon-Wiener index of shrubs and herbaceous showed exactly the same trend that fluctuated at early stage, then declined at medium stage, at last increased and became stable at mature stage. There were less common species between the communities of early age (3a, 5a, 8a), however, there were more common species between the communities of mature stage(37a, 39a) which increased the similarity coefficient of understory plant community, the maximum similarity coefficient reached 62.1%, showing that level of similarity of understory plant species were closer and the ecosystem habitats were tend to be stable of the mature plantations.