Farmland soil organic carbon pool is one of the most important soil carbon pools in terrestrial ecosystem. Clarifying the spatial distribution characteristics and influence mechanism of farmland soil organic carbon can provide sampling and theoretical basis for the evaluation of soil fertility and carbon sequestration capacity. Taking a typical agricultural irrigation and drainage unit (area: 45 hm²) in the Jianghan plain as the research object, the soil organic carbon contents (SOC) of 1 560 soil samples in 104 sites with the depth of 0—200 cm were measured, and the soil carbon density (SOCD) was calculated. We revealed the spatial characteristics and the distribution patterns of soil organic carbon density, and analyzed the influence of cultivation mode and history on its distribution. The results showed that: (1) In the 0—200 cm soil profile, the mean variation of SOCD in the 20 cm and 200 cm soil layer was 1.75~3.77 kg/m² and 11.67~34.24 kg/m², and the SOCD decreased sharply at first and then increased slowly as the increase of soil depth. The SOCD in 100—200 cm soil layer accounted for about 45.26% of the whole soil profile. Therefore, we should pay more attention to the deep-layer SOCD. (2) The SOCD in 0—20 cm and 0—200 cm soil layer under an Irrigation-Drainage Unit had strong spatial autocorrelation, indicating that structural factors such as soil parent material and topography were the dominant factors affecting the spatial distribution of SOCD under an Irrigation-Drainage Unit. (3) The SOCD among different cultivation modes and history were significantly different. It showed that the SOCD of all soil layers in paddy field was higher than that of upland field. The SOCD of was1.31 times that of upland field in the 0—200 cm, and the old paddy field was higher than the new paddy field in all soil layers. However, the difference between the total SCOD of the woodland to paddy field and the SCOD of the dryland to paddy field was relatively small in the 0—200 cm, while the higher SOCD was observed in woodland than dryland. The above shows that increasing the area of paddy field is one of the effective ways to rapidly increase farmland carbon storage. (4) The representative stable depth of SOCD at the scale of irrigation and drainage units was 180—200 cm, and the sampling depth should be extended as deep as possible when we investigated SOCD. These results provide a scientific basis for improving the accuracy of estimation of SOCD and the rationality of sampling design, as well as the evaluation of soil carbon sequestration capacity.