Abstract: Undergrowth of plants in a Chinese fir plantation in different growth stages and site indices has been quantitatively analysed by using β-diversity index formulas. The results showed that the values of β-diversity index of undergrowth plant in different growth stages and site indices are obviously different , in which the difference between 56-year-old and 32-year-old forestsis greatest , and the closer to 56-year-old in growth stages , the smaller in β-diversity index values , in each Chinese fir community , the difference of β-diversity index values of undergrowth plant between 14 and 18 site indices is also great .