A research using the Denitrification-Decomposition (DNDC) model was carried out to study spatial-temporal variation of SOC storage at both field and county scales, based on the data of a long-term (12 a) fertilization experiment in paddy field and the comparison of two field surveys in 1998 and 2001 in Yujiang County. Results show that the model simulated well the variation of SOC in the long-term experiment. The total SOC storage in the surface layer (0~20 cm) of paddy field in Yujiang County was estimated at 2.9×10⁹ kg and the average SOC density was 6.0 kg m^-2 in 2001. The SOC storage in the surface layer increased annually on average by 3.0×10⁷ kg from 1998 to 2001. Scenario analysis of the period from 1998 to 2017 show that SOC storage could be effectively improved by planting green manure, increasing straw returning rate and reducing chemical fertilizer application rate.