[Objective] Explore the characteristics of soil total organic carbon and heavy fraction organic carbon and their relationship to provide scientific bases for studying the carbon fixation effect and soil stability in Ningxia irrigation zone.[Methods] Taking Ningxia irrigation zone as the study area, the contents of total and heavy fraction organic carbon of 5 different types of soil were measured.[Results] Soil type is the main factor for change of soil organic carbon in the Ningxia irrigation zone. After different durations of irrigation and cultivation, total and heavy fraction organic carbon of irrigated soil were both increased. As compared with the non-irrigation and non-cultivation soil, the average increments were 2.27 and 2.02 g/kg, respectively. The increments decreased with the increase of soil depth, but the amplitudes differed among different soil types. There existed significant correlation between fraction organic carbon and total organic carbon.[Conclusion] Heavy fraction organic carbon is an important part of total organic carbon, its correlation with total organic carbon increase with the increase of soil depth. Irrigation and cultivation activities and irrigation time have influence on the soil organic carbon. Studying the fraction organic carbon can better understand soil carbon and soil stability.