Abstract: Ecological problems arising from under-forest economy have gained a wide recognition in recent years. With the development of under-forest economy, understory farming has become more sophisticated, and ecological environments under forests have also become more stable in Rongchang region of Chongqing. The adjacent-sample comparison method was used to study the effects of six different typical under-forest economy modes on importance value (P), species richness index (S), Shannon-Wiener diversity (H′), Pielou’s evenness index (J_w), Alatalo dominance index (E_a), Jaccard similarity index (C_j), Sorenson similarity index (C_s) and plant species diversity threshold (D_v) in China’s Southwest Mountains from August to September in 2012. The investigated six under-forest economy modes were poultry feeding in Eucalyptus robusta forest (T1), fungi cultivation in E. robusta forest (T2), poultry feeding in Dendrocalamus latiflorus forest (T3), fungi cultivation in D. latiflorus forest (T4), livestock feeding in Pinus massoniana forest (T5) and grass cultivation in Ficus lacor forest (T6), and with the corresponding pure forests as the controls. The results showed that among six under-forest economy modes, the highest species richness index (11) was observed in T6 and the lowest species richness index (2) was occurred in both T3 and T4. Herb layer was the most dominant layer for all the six under-forest economy modes, and no shrub layer was found. The highest importance values were of Alternanthera philoxeroides in T1 (67.16%) and T2 (71.00%), Morus alba in T3 (74. 91%) and T4 (72.82%), and Oplismenus compositus in T5 (54.10%), and Hemarthria altissima in T6 (59.51%). For six under-forest economy modes, the species richness index was in the order of T6 > T1 > T5 = T2 > T3 = T4, the Shannon-Wiener diversity was T6 > T1 > T5 > T2 > T4 > T3, the Pielou’s evenness index was T5 > T6 > T2 > T1 > T4 > T3, and Alatalo dominance index was T5 > T4 > T3 > T2 > T1 > T6. Based on the analysis of similarity of species composition, T3 and T4 had the highest similarity indexes with their corresponding pure forests, 1.0 of both C_j and C_s, followed by T6 with 0.44 C_j and 0.62 C_s, respectively. The minimum C_j (0.15) and C_s (0.27) were those of T1 with its pure forest. Based the evaluation of plant diversity threshold under different under-forest economy modes, T6 had the highest threshold with a better diversity. Then the lowest threshold was for T3 and T4. Understory farming altered species composition of plant community, and different farming methods had different results. Grass cultivation under forest had the least effect and also was the most abundant species above all inquisitional modes.