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Introduction Grassland degradation in karst regions is typically characterized by increased bedrock exposure, vegetation fragmentation, and soil structural instability. However, the mechanisms by which these changes affect hillslope erosion resistance remain poorly quantified. Methods In this study, natural grassland plots under four levels of degradation, defined by bedrock exposure rates (RER) of 0%, 20%, 40%, and 55%, were selected in representative limestone and dolomite areas in Guizhou Province, China. Undisturbed soil samples from the 0∼20 cm layer were collected for aggregate stability tests and undisturbed soil scouring experiment. Using 9 diagnostic indicators of soil structural function, the effects of degradation on soil retention capacity were quantitatively assessed. Results The results showed that with increasing RER, the proportion of macroaggregates (>2 mm) decreased by 31.6%, and mean weight diameter (MWD) declined by 58.3%. Relative dispersion index (RSI) and relative mechanical Breakdown index (RMI) increased to 1.96 and 2.21, respectively, with the most severe structural breakdown occurring under fast wetting conditions. In the scouring experiments, sediment concentration peaked at 1.8 g/min within the first minute in the 55% RER plots, significantly higher than in the 0% RER plots. Meanwhile, the soil resistance coefficient declined by more than 50%. Composite functional evaluation revealed that MWD, RSI, Anti-scourability coefficient (AS), and root surface area were the most sensitive indicators across degradation levels. Limestone grassland (LG) demonstrated stronger performance in maintaining structural integrity and erosion resistance compared to Dolomite grassland (DG). Discussion These findings provide a scientific basis for identifying early warning signs of erosion resistance loss and offer theoretical support for ecological restoration and degradation threshold identification in karst grassland ecosystems.