Abstract: Aggregates are the basic units of soil structure and are important factors affecting soil quality. Their formation is influenced by agricultural management practices and environmental conditions, leading to variation in structure across different regions and soil types. Brown soil is one of the main soil types in Hebei Province and understanding the influence of long-term wheat-maize rotation on the formation of soil aggregates is essential for improving soil quality and enhancing grain storage in the ground. This study was based on a long-term experiment involving straw return and nitrogen application in a brown soil region, aiming to determine their effects on the water stability and composition of soil aggregates, as well as the soil organic carbon (SOC) and total nitrogen (TN) content at different particle sizes. Four treatments—straw returning without nitrogen fertilizer (CK), traditional nitrogen fertilizer with straw returning (CON), optimized nitrogen fertilizer with straw returning (OPT) and traditional nitrogen fertilizer with straw removal (CON-S)—were selected to study the effects of nitrogen application and straw return on the water stability of brown soil aggregate composition and soil organic carbon (SOC) and total nitrogen (TN) content at different particle size levels. Four types of aggregates with different particle sizes were obtained using the wet-screening method. The SOC and TN contents within different aggregates sizes were measured, and the composition and stability of water-stable aggregates under different treatments were analyzed. Additionally, the contribution of each aggregate size class to overall SOC and TN content was calculated to evaluate their respective roles in nutrient distribution. The results showed that soil pH and calcium carbonate content were positively correlated with the stability of soil aggregates, which significantly promoted the formation of soil water-stable macroaggregates and improved the stability of water-stable aggregates; however, nitrogen application had no significant effect on the stability of water-stable aggregates under straw return. Under the same nitrogen application conditions, the SOC content of the soil aggregates was significantly increased by straw return, and the SOC content of the water-stable soil aggregates was significantly increased by nitrogen application. Nitrogen application significantly increased the TN content of aggregates at all levels, and straw return significantly increased the TN content of water-stable aggregates at all levels under the same nitrogen application conditions. The SOC and TN contents in the water-stable aggregates decreased with decreasing water-stable aggregate particle size. Compared with straw removal, straw return significantly increased the contribution rates of SOC and TN in water-stable aggregates >2 mm and 0.25−2 mm, while reducing their contribution in the 0.053−0.25 mm and < 0.053 mm fractions. However, nitrogen application had no significant effect on the SOC and TN contributions of the water-stable aggregates. There were no significant differences in the water-stable aggregate composition and SOC and TN contents between optimal and traditional fertilization treatments. In summary, in brown soil areas, soil stability and fertility can be improved by optimizing nitrogen application under straw return measures.