A study was conducted in Minqin Oasis of wuwei city in North-West China to understand the trend of soil organic carbon fractions and its influencing factors after several years of alfalfa planting in the Minqin Oasis. Three treatments were conducted, including 3 years (A3), 14 years (A14) and 22 years (A22) continuous cropping of alfalfa. Content of total organic carbon (TOC), heavy fraction organic carbon (HFOC), light fraction organic carbon (LFOC), particulate organic carbon (POC), and microbial biomass carbon (MBC) in the soil were determined through field survey sampling and indoor laboratory analysis. The results showed that soil bulk density and electrical conductivity of 22 years alfalfa soil decreased by 13.9% and 95.4%, respectively, compared with that of cotton soil. Compared with the cotton soil (CK), the TOC, LFOC, HFOC, and MBC of 3 years, 14 years and 22 years alfalfa soil was significantly increased by 175.0%, 1 416.0%, 47.8%, and 216.0%, respectively. Vertical profile of cotton soil showed that content of LFOC, POC, MBC decreased with the increase of soil depth. Stepwise regression analysis showed that there was a significant positive correlation between LFOC and TOC (p＜0.05). Alfalfa planting age, soil layer and their interaction had a significant impact on POC/TOC (p＜0.01), and an extremely significant effect on TOC, LFOC, HFOC, POC, MBC, LFOC/TOC, HFOC/TOC, and MBC/TOC (P＜0.001). Alfalfa planting could improve soil quality and reduce soil bulk density and salinity. The study revealed that continuous planting for 14 years is the most reasonable planting time for Alfalfa. Meanwhile, Alfalfa planting is beneficial to the accumulation of organic carbon fractions in the 0—60 cm soil layer, especially to the increase of TOC, LFOC, and MBC.