Fertilization affects the effect of straw returning to the field. To study the effects of different forms of nitrogen fertilizer on the decomposition of straw and the soil organic carbon fractions can provide scientific basis for the rational application of nitrogen fertilizer under straw returning. Based on the 10-year experiment of wheat-maize rotation and location fertilization, the effects of different amounts of controlled-release urea mixed and common urea(CRF)and common urea (CF) (120, 240 and 360 kg N/hm²) on the decomposition of maize straw and soil organic carbon fractions in fluvo-aquic soil were studied. The results showed that compared with the CF treatment, the CRF treatment had a tendency of promoting the decomposition of straw. At the late stage of straw decomposition, CRF treatment significantly promoted the release of nitrogen and phosphorus from straw compared with CF treatment, while different nitrogen fertilizer treatments had no significant effect on the release of potassium from straw. At the later stage of straw decomposition, under the same nitrogen application rates, CRF treatment with conventional nitrogen application and increased nitrogen application significantly increased the content of water-soluble and heat-soluble organic carbon in soil. In terms of increasing soil microbial biomass carbon and nitrogen contents, the promotion of CRF treatment was significantly higher than that of CF treatment at some stages of straw decomposition. In general, compared with CF treatment, CRF treatment has a better effect on promoting straw decomposition and increasing soil organic carbon fractions.