Abstract:The wet sieving and density fractionation scheme was used to study the content and evolution characteristics of organic carbon fractions in grey desert soil under 18 years‘ fertilization. The results showed that, compared with CK, manure combined with inorganic fertilizer（NPKM and 1.5 NPKM）was most effective on the increase the all organic carbon fractions, and also kept them in the highest increase rate. The average increase rate in coarse free particalate organic carbon(cfPOC) was 0.12 g/(kg·a), in fine free particulate organic carbon(ffPOC) was 0.06 g/(kg·a), in intra-microaggregate particulate organic carbon(iPOC) was 0.08 g/(kg·a)and in mineral-associated organic carbon(MOC) was 0.17 g/(kg·a), respectively. Straws return also increased cfPOC and ffPOC by 0.05 and 0.03 g/(kg·a)respectively. The treatment of abandonment(CK0)and chemical fertilizer maintained the organic carbon content in all fractions. There was a significantly positive relationship between the different organic carbon fractions. The cfPOC under long term fertilization was increased higher than other organic carbon fraction by 2.1～8.0 times, which implied cfPOC was more sensitive to the fertilizations. The MOC, accounting for 56.9%～77.8% of total organic carbon(TOC), was main form for organic carbon sequestration in grey desert soil. In a word, long-term manure combined with chemical fertilizers was benefit to enhance content of organic carbon fractions and improve fertility of gray desert soil.