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Soil aggregates are the basic units of soil structure. Water-stable aggregates (WSAs) are greatly influenced by tillage practices. The main objective of our study was to assess the effects of different tillage practices on water aggregate stability on hillslope cropland in northern China and to identify the relationship between soil aggregates and soil structure stability. The study was conducted under on-farm conditions. Soils from plots with no tillage (NT), rotary tillage (RT), and conventional tillage (CT) treatments were sampled for 2 years (2016–2017) in the study area (Xiangyang village, Harbin city, Heilongjiang Province). Soil aggregates were collected, and the mean weight diameter (MWD) and geometric mean diameter (GMD) were calculated. The relationships between the soil aggregate variables and soil structural stability were determined using multivariate stepwise analysis. The results indicated that for macroaggregates, the MWD and GMD increased under NT and more readily formed WSAs > 5 mm. The effects of the different treatments on microaggregates were similar among the different slope positions. The variation in the proportion of macroaggregate across all the treatments was lower in 2017 than that in 2016. The reduction in soil macroaggregates under NT was much smaller than the reductions under RT and CT, suggesting that soil structure can be maintained under NT. The multivariate stepwise analysis showed that the proportion of WSAs > 5 mm were positively and significantly correlated with MWD and GMD and represented the predominant factor influencing soil structure according to the discriminant coefficients. The proportion of microaggregates also affected the MWD and GMD of the soil aggregates but had negative correlations with these variables. There were significantly positive correlations between the soil organic matter content and soil structure stability variables (macroaggregate content, MWD, and GMD). Overall, macroaggregates in soil likely play a key role in soil structure stability. The soil aggregate stability is also affected by slope position. Comparing RT and NT, soil structure can be better maintained with NT.