Abstract: Aggregate is the basic unit of soil structure, which is an important factor affecting soil quality. The formation mechanism of aggregates is influenced by agricultural management practices and environmental factors, therefor the structural of Aggregate in different regions and in different soil types. Brown soil is one of the main soil types in Hebei Province, It is of great significance to understand the influence of long-term wheat-maize rotation on the formation of soil aggregates for improving soil quality and realizing grain storage in the ground This study was based on the long-term experiment of straw returning and nitrogen application in the brown soil area, in order to determine the effects of nitrogen application and straw returning on the water stability of brown soil water-stable aggregate composition and soil organic carbon (SOC) and total nitrogen (TN) contents at different particle size levels. Four treatments CK (straw returning without nitrogen fertilizer), CON (traditional nitrogen fertilizer with straw returning), OPT (optimized nitrogen fertilizer with straw returning) and CON-S (traditional nitrogen fertilizer with straw removal) were selected: to study the effects of nitrogen application and straw returning 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 kinds of aggregates with different particle sizes were obtained by wet screening method. SOC and TN contents in different aggregates were detected, and the composition and stability of water-stable aggregates under different treatments and the contribution rates of water-stable aggregates at different levels to soil SOC and TN were calculated, as well as the contribution rates of water-stable aggregates at different levels to SOC and TN. 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, but nitrogen application had no significant effect on the stability of water-stable aggregates under the condition of straw returning. Under the same nitrogen application condition, the SOC content of soil aggregates was significantly increased by straw returning, and the SOC content of soil water-stable aggregates was significantly increased by nitrogen applying. Nitrogen application significantly increased the TN content of aggregates at all levels, and straw returning significantly increased the TN content of water-stable aggregates at all levels under the same nitrogen application condition. The content of SOC and TN in water-stable aggregates decreased with the decrease of water-stable aggregate particle size. Compared with straw removal, straw returning significantly increased the contribution rates of SOC and TN in water-stable aggregates > 2 mm and 0.25−2 mm and decreased the contribution rates of water-stable aggregates 0.053−0.25 mm and < 0.053 mm. However, nitrogen application had no significant effect on SOC and TN contributions of water-stable aggregates. There were no significant differences in water-stable aggregate composition and SOC and TN contents between optimal fertilization and traditional fertilization. In summary, in the brown soil area, soil stability and fertility level can be improved by optimizing nitrogen application under straw returning measures.