The restoration and reconstruction of desertification grassland in arid and semiarid region are critical constituents of regional ecological construction. Changes of unprotected soil organic carbon in different desertification stages of desert grassland is important in discussing the mechanism of desertification grassland. The characteristics of the content and proportion of coarse particulate organic carbon (CPOC), fine particulate organic carbon (FPOC), light-fraction organic carbon (LFOC) and the rate constant for C transferring from the unprotected to the protected soil pool (k) in different desertification stages of desert grassland in arid and semiarid region were studied. The results demonstrated that during the desertification process of desert grassland, the rank of the content and proportion of FPOC was fixed dunes >grasslands > semi-fixed dunes > mobile dunes, the rank of the CPOC content was grasslands > semi-fixed dunes > mobile dunes > fixed dunes; and the rank of the CPOC proportion was mobile dunes > semi-fixed dunes > grasslands > fixed dunes. The LFOC content and proportion decreased gradually during the desertification process of desert grassland. Particulate organic carbon (POC) was more sensitive to desertification grassland than LFOC and SOC, and FPOC was more sensitive than CPOC, LFOC and SOC. The proportion of unprotected soil organic carbon decreased gradually during the desertification process in desert grassland, which indicated that there was a decrease in soil quality. The rate constant for C transferring from the unprotected to the protected soil pool (k) increased gradually during the desertification process in desert grassland (from grasslands to mobile dunes).