This paper studied effects of dryland terrace and crop types on soil organic carbon density in the watershed in northern semiarid area.It provided theoretical guidance for reasonable farming and ecological safety in small watershed.25 sample plot and 125 soil profiles in Dawujia small watershed in Aohan county of Chifeng in Inner Mongolia were selected in this study.The following results were obtained:(1)Soil organic carbon density for maize,sorghum,millet and mung bean decreased with soil depth and then increased,the maximum soil carbon density was in the depth of 50～100 cm；(2)The soil organic carbon density for all the four kinds of crops were reached the maximum at the elevation of 400～500 m areas,followed by an altitude of more than 600 m,and it was the lowest at the elevation of 500～600 m region；(3)The maximum soil organic carbon density of four kinds of crops was in the areas of 2～6°,and the minimum soil organic carbon density was in the areas of≤2°;(4)In the level of P<0.05，the soil organic carbon density for four kinds of crops changed different in different slope.Soil organic carbon density for maize and millet showed a trend of south slope＞north slope＞west slope＞east slope,and the sorghum and mung bean the trend was north slope＞south slope＞west slope＞east slope;(5)The soil organic carbon density of mung bean was higher than other crops,with an average soil organic carbon density of 0.81 g·cm-2.