Deep application of straw (DAS) refers to a technique of incorporating crushed corn stalk into the subsoil layer (20~40 cm). It may effectively improve straw utilization rate, reduce straw burning rate, extend the extent of soil building from previously topsoil down to subsoil, contribute to accumulation of organic matter in the subsoil layer, help solve problems like thinning of the plow layer, shallowing, thickening and hardening of the plow pen, and lacking organic matter, and achieve the ends of higher soil fertility, higher water holding capacity, and higher crop yield. There used to be quite a number of researches focused on effect of DAS on soil physical properties, and few on soil chemical properties, let alone its effect on humus composition in soil aggregates. This paper mainly studies changes in structure of humic acid (HA) in soil aggregates in the topsoil and subsoil layers caused by DAS, so as to provide some theoretical basis for solutions to how to improve soil fertility and build up a proper plow layer. Soil samples were collected from an experimental mono-cropping corn field which had two plots, DAS and non DAS, in the Jilin Agricultural University. Four factions of soil aggregates, >2 mm, 2～0.25 mm, 0.25～0.053 mm and<0.053 mm were screened out of the samples separately, with the wet sieve method and HA extracted for analysis of element composition with infrared spectroscopy and differential thermal analysis. Comparison was performed to determine effects of DAS on HA composition. Results show that the black soil tested was dominated with the fraction of 2~0.25 mm soil aggregates; however, the content of the dominant fraction and the content of organic carbon in the topsoil was lower than their respective ones in the subsoil; DAS was conducive to formulation of soil aggregates of the dominant fraction and increased the organic carbon content in the soil aggregates. Compared with that in the subsoil, the HA in soil aggregates, regardless of fractions, in the topsoil was lower in condensation degree and oxidation degree and thermal stability, and higher in aliphatic-C/carboxylic-C (2 920/1 720) and aliphatic-C/aromatic-C (2 920/1 620) ratios. DAS was found to reduce condensation degree, oxidation degree and thermal stability of HA in all soil aggregates in the topsoil and subsoil layers, especially HA condensation degree in the topsoil. On the other hand, oxidation degree and thermal stability of the subsoil layer also decreased obviously. The HA in the topsoil was simpler in structure and younger than that in the subsoil layer. So, DAS may also promote accumulation of organic carbon in the soil and make HA simpler and younger in structure.