To investigate the effects of plant communities in different forest types on soil quality in ecologically fragile karst areas, as well as the key factors influencing soil quality, this study focused on various forest types—including primary, secondary, and planted forests—in the karst mountains of Baise, Guangxi. The study examined the characteristics of plant communities under different forest types and collected soil samples from various soil layers (0–30 cm, 30–60 cm, and 60–90 cm). The physicochemical properties of the soil, enzyme activities, and microbial biomass carbon, nitrogen, and phosphorus were measured. Using analytical methods such as principal component analysis, a minimum data set (MDS) was constructed, and the soil quality index (SQI) was calculated to evaluate soil quality. Key factors influencing the soil quality index were also analyzed. The results indicate that total organic carbon (TOC) content, nutrient levels, microbial biomass, and enzyme activity were significantly higher in primary and secondary forests than in plantations. A minimal dataset comprising six indicators—total phosphorus (TP), total organic carbon (TOC), total potassium (TK), available potassium (AK), pH, and acid phosphatase (ACP)—effectively characterizes soil quality. Soil quality across different plant communities was ranked as follows: Liquidambarformosana - Oroxylum indicum community > Cunninghamia lanceolata community > Triadica cochinchinensis - Phyllanthusemblica community > Liquidambar formosana community > mature Eucalyptusgrandis × urophylla plantation community > young Eucalyptusgrandis × urophylla plantation community. Across different soil depths, soil quality was higher in the surface layer than in the deeper layers. The understory shrub and herb layers are key factors influencing soil quality; optimizing community structure and promoting the recovery of understory vegetation play a crucial role in improving soil quality. This study provides a scientific basis for soil quality monitoring, evaluation, and vegetation restoration management in ecologically fragile karst areas.