This paper studied the effects of 1-, 2- and 3 years of planting Pennisetum sp. on the plant- and insect diversity in the Pennisetum sp. communities, taking the barren mountain land without planting Pennisetum sp. as the control (CK). Compared with CK, the plant species richness in Pennisetum sp. communities with different years of planting was lower, but the coverage was higher. The coverage in the Pennisetum sp. community having been planted for 3 years was the highest, up to 91.6%, and 75.8% higher than the CK. The insect species richness in the Pennisetum sp. communities having been planted for 1, 2 and 3 years was 3.6, 5.3 and 5.6 times of the CK, respectively. The plant- and insect diversity indices, including Simpson index, Shannon index, evenness, Brillouin index, and McIntosh index for the Pennisetum sp. communities with different years of planting were significantly higher than the CK, which indicated that the growth of Pennisetum sp. could affect the plant- and insect diversity. With the increasing year of planting, the plant- and insect diversity in Pennisetum sp. communities tended to be stable.