Based on the vector patch units of the 1:50 000, 1:1 000 000 and 1:14 000 000 soil maps of the Tai Lake region and the grid units, different in resolution, derived from the above mentioned vector patch units, and the 1982 paddy soil organic carbon pool of the region as well, soil organic carbon pool in the surface layer (0~20 cm) of paddy fields in the region of the year of 2000 was simulated using the DNDC model. And comparison analysis was done of variations of number of soil types, area of each soil type, soil organic carbon storage in the surface layer and soil organic carbon density relative to grid units different in solution and vector patch units different in scale. The 4 indices derived from the vector patch units were set as benchmarks and the relative variability (VIV, %) used to discriminate optimal resolutions of grid units simulated based on the DNDC models of different scales. Results show that the optimal resolution of the simulated grid units converted from the vector patch units of the 3 different scales is 0.2 km × 0.2 km, 2 km × 2 km and 17 km × 17 km, separately, on the premises that all the four indices are less than 1% in VIV. The optimal resolution of grid units can not only meet the requirement for precision in simulation, but also avoid data redundancy and waste of resources. Relationships established between the scales of the soil organic carbon simulation researches and the optimal resolutions of grid units are of great reference value to the researches on modeling of soil organic carbon pool on a regional scale.