Five plantations (Ⅰ: Cupressus funebris and Pterocarya stenoptera forest; Ⅱ: Ginkgo biloba and Phoebe zhennan forest; Ⅲ: Swida wilsoniana and Cinnamomum camphora forest; Ⅳ: Pt. stenoptera and Alnus cremastogyne forest; Ⅴ: Cu. funebris forest) were utilized in the Yunding Mountain, Chengdu, as the research objects to focus on the differences between understory plant diversity and soil physicochemical properties and the relationship between them. The results showed that the species with the highest important values in the shrub and herb layer of five plantations were different. S. wilsoniana and Ci. camphora forests had the richest understory plant composition whereas G. biloba and Ph. zhennan forests had the poorest. In the shrub layer, the Shannon-Wiener diversity index H, Simpson dominance index H', and species richness index D of the different plantations were the largest in the S. wilsoniana and Ci. camphora forests, whereas the Pielou evenness index J_sw showed no significant difference. In the herb layer, the H, D, and J_sw among the different plantations were the largest in the Cu. Funebris forest and the smallest in the G. biloba and Ph. zhennan forests, whereas the difference of the H' in the different plantations was not significant. There were visible differences in the soil pH value and contents of organic matter, total nitrogen, total phosphorus, available potassium, and available phosphorus in the different plantations. The pH value and total phosphorus contents were the highest in the Cu. funebris and Pt. stenoptera forests, and the organic matter, total nitrogen, available potassium, and available phosphorus contents were the highest in the G. biloba and Ph. zhennan forests. The soil factors that significantly affected the species diversity of the shrub layer were the organic matter and available phosphorus, whereas in the herb layer these factors were the pH, total phosphorus, and available phosphorus. In summary, the overall levels of understory plant diversity under S. wilsoniana and Ci. camphora forests were the highest; therefore, the construction of such a plantation is conducive to the maintenance and development of understory plant diversity. Meanwhile, soil pH, organic matter, total phosphorus, and available phosphorus were the main soil factors influencing the understory plant diversity in the study area.