The mechanism of soil aggregate fragmentation under natural conditions was simulated in order to study the changes of aggregate stability with different straw returning methods and determine the soil structural characteristics and the relationship between organic carbon and aggregate stability. The results would provide theoretical basis for implementing reasonable erosion prevention and control techniques on sloping farmland of black soil areas. The results showed that the changes in soil aggregate stability under different straw returning measures were influenced by the fragmentation mechanism and returning methods , and fast wetting was the main mechanism for aggregate fragmentation. There was a significant difference in MWD values with fast wetting and wet stirring between aggregates of <2 mm and 2-5 mm, the aggregates stability of 2-5 mm particle in wet stirring was the highest, which indicating that higher stability was large particle aggregates in black soil and soil organic carbon was positively correlated with the stability of large particle aggregates. Size aggregates of <2 mm particle were significantly positively correlated with soil bulk density and capillary pores under moist stirring, while significantly negatively correlated with total porosity and non capillary pores. In short term the non biological factors such as soil bulk density and porosity had masked the role of straw mulching in preventing surface erosion and stabilizing aggregates. However the total porosity and non capillary porosity of straw mixed and returned soil were decreased and the stability of aggregates was shown an increasing trend.