Soil organic matter, as a basic substance of soil fertility, is not only a major source of soil nutrients, but also a critical factor controlling soil physic-chemical properties, biological properties and various fertility factors. In China and other countries, soil organic matter in farmland is maintained and raised mostly through incorporation of crop straw. Humification processes of the straw incorporated, and accumulation of organic carbon, composition of soil humus and soil enzyme activities varied sharply between soils different in depth of straw incorporation, and also between soils different in straw incorporation rate. To verify the effects of depth and rate of straw incorporation on these factors. a field experiment was carried out in a tract of farmland of black soil. In the field experiment, corn stalk packed at a rate of 40 g kg^-1 and 80 g kg^-1 soil in nylon bags was buried into three soil layers different in depth (0 ~ 20, 20 ~ 40 and 40 ~ 60 cm) for 4 years of in situ incubation for monitoring variations of corn stalk transformation, organic carbon accumulation, humus composition and soil enzyme activities. Results show that the addition of 40 g kg^-1 soil of the straw increased organic carbon by 31.8%, 96.4% and 171.1%, respectively, at the 0 ~ 20, 20 ~ 40 and 40 ~ 60 cm soil layers,, and the addition of 80 g kg^-1 soil of the straw did by 86.2%, 193.5% and 265.9 %, respectively; demonstrating that depth of the incorporation is conductive to soil organic carbon accumulation, because poor aeration in deep soil layers slows decomposition and transformation of the straw, thus favoring humus accumulation. However, in control (no straw incorporated), organic carbon declined by 29.3% in the 0 ~ 20 cm soil layer, but only by 1.8% in the 20 ~ 40 cm soil layer . The reason is that soil enzyme activity is higher in the shallow soil layer than in the deep soil layer, and hence, soil humus mineralization rate is much higher in the former than in the latter. Higher straw incorporation rate (80 g kg^-1 soil) increased soil organic carbon and soil enzyme activity. As a result humus HA / FA ratio in the soil changed greatly, towards improving humus in quality. Accumulation of humine acid carbon (HS-C) and humic acid carbon (HA-C) varied with soil depth, exhibiting a decreasing order of 20 ~ 40 cm > 0 ~ 20 cm > 40 ~ 60 cm, while accumulation of fulvic acid carbon (FA-C) in the 40 ~ 60 cm soil layer was more prominent. The contents of HA-C and FA-C were in extremely significant positive relationship with catalase activity and in significant positive one with urease and invertase activities. The above-listed findings suggest that incorporation of a high amount of straw may quickly increase soil organic carbon content, and deeper incorporation contributes to accumulation of organic carbon. Straw incorporation also alters soil humus composition and promotes activities of catalase, urease and invertase. For comparison, after the four years of experiment, the control was found to be much lower in soil organic matter content in the surface layer, which indicates that renewal of soil organic matter in farmland is very important to maintenance of soil fertility.