Abstract: The effect of thinning intensities with four different treatments of 75.25%(Ⅰ), 71.85%(Ⅱ), 67.48%(Ⅲ) and 58.75%(Ⅳ) on the undergrowth were carried out in the 23-year-old stands Larix principis with initial density of 5151 trees/hm2 , in lower hilly lands of Yingpan, Yanqing Country of Bejing Municipality. In 2005, 5 years after thinning, the composition, abundance and diversity of the undergrowth in the four plots were investigated and compared. The results showed that both the numbers of species and the composition of the dicotyls were enhanced by thinning density. Meanwhile, the life-form spectrums were influenced and the percentage of chamaephytes and geocryptophyte went as follows: Ⅰ<Ⅱ<Ⅲ<Ⅳ, opposite to the trend of the phanerophytes. Also, the composition of the dominant species of the undergrowth varied greatly, which of the herb layer responded more than shrub layer. The abundance of undergrowth was improved more greatly than evenness by thinning intensities. Additional, the shrub layers were more influenced than the herb layers in the all plots. Based on the results, it was concluded that reasonable thinning intensity was benefit to the development of the undergrowth and so can be an effective way for the sustainable use of plantations. Generally, when the stand with initial density of 5151trees/hm2 grows up to 23 years old, the reasonable thinning intensity should be about 58.75%.