To explore the suitable application rate and the ecological effect of Chinese milk vetch replacing chemical fertilizer, an eleven-year field experiment was conducted to investigate the annual variations of active organic carbon(AOC) concentrations under different replacing rates of Chinese milk vetch. Soil samples were collected in the typical periods of rice-rice-Chinese milk vetch rotation system, i.e., before the incorporation of Chinese milk vetch, tillering stage of early rice, mature stage of early rice, tillering stage of late rice and mature stage of late rice, and the contents of soil total organic carbon (TOC), AOC, microbial biomass carbon (MBC) and dissolved organic carbon (DOC) were measured. The results showed that:compared with CK treatment, Chinese milk vetch replacing 40% chemical fertilizer increased TOC. Compared with the 100% chemical fertilizer treatment, the treatments of Chinese milk vetch replacing chemical fertilizer increased AOC, MBC and DOC to varying degrees. AOC, MBC and DOC increased first then decreased with the increased amount of Chinese milk vetch. AOC was highest with 30.0 t/hm² of Chinese milk vetch application, MBC and DOC were highest with 22.5 t/hm² of Chinese milk vetch application. The sampling period had a great influence on MBC, DOC, MBC/TOC and DOC/TOC, which showed decreasing-increasing-decreasing tendency. At the mature stage of late rice with relatively stable soil properties, AOC/TOC, MBC/TOC and DOC/TOC increased with the increase of Chinese milk vetch in a certain range, beyond this range, no significant change was found in AOC/TOC and MBC/TOC, and DOC/TOC decreased significantly (P<0.05). Comprehensively, reducing chemical fertilizer with the long-term planting Chinese milk vetch is beneficial to increase soil active organic carbon content and proportion, and the optimal application rate of Chinese milk vetch is 22.5-30.0 t/hm².