Soil erosion caused by water is a major form of soil degradation. Scientists suggest that this phenomenon can be predicted based on aggregate stability. However, most of these scientists have focused on the water stability of aggregates; only a few have devoted efforts to study the mechanical stability of aggregates. To investigate the relationship of aggregate breakdown with mechanical stability and the possibility of predicting soil erosion based on mechanical stability, we collected eight kinds of Ultisol aggregates (3–5 mm) from subtropical China and exposed these aggregates to simulated rainfall (60 mm h−1). Mean weight diameter (MWD) and splash loss after rainfall were analyzed and compared with aggregate water stability and mechanical stability. Aggregate water stability was determined by wet sieving methods; aggregate mechanical stability was tested by aggregate penetration resistance and tensile strength. Wet sieving method, tensile strength, and penetration resistance of dry aggregates exhibited a good relationship with aggregate MWD (R2 ≥ 0.838, 0.775, and 0.593, respectively) under rainfall and splash (R2 = 0.758, 0.865, and 0.563, respectively). In contrast, no relationship was found between penetration resistance of wet aggregates and MWD or splash. Thus, MWD of wet sieving method and tensile strength were the better indexes that could be used to estimate fragment size distribution and splash detachment. Tensile strength also correlated with clay contents (r = 0.793*), Fed (r = 0.735*), and Feo (r = 0.911**). With further studies, a simpler and faster method could be developed to estimate soil resistance to erosion based on tensile strength in this region.