Soil quality index (SQI) is an important tool for evaluating soil quality (SQ). Studies on the effect of understory vegetation on soil properties and SQ in different seasons under rubber plantation is limited. With this aim, we selected four sites (Bubeng, Jingtai, Nabanhe National Nature Reserve and Xishuangbanna Tropical Botanic Garden) in Xishuangbanna, southwestern China. Soil samples were collected during dry and rainy seasons at 010 cm depth and 16 soil properties were analysed. SQI was computed from the minimum data set derived from principal component analysis. Results show that low plant diversity significantly reduced the nematode abundance in all four study sites. Seasonality (dry and rainy) and plant diversity (high and low) had more impact on the arthropod population. Total organic carbon, total nitrogen, arthropod abundance, the mass of dry root, nematode abundance and total sulphur were identified as key soil quality indicators, where total nitrogen (36.3%) and total organic carbon (30.5%) contributed the maximum to SQ. SQI values showed that SQ significantly decreased from high to low plant diversity in its corresponding seasons. Thus, our results recommended modifying management practices by conserving natural understorey vegetation, in turn, enhance the soil nutrients and soil quality in the monoculture rubber plantation.