Two kinds of paddy soil with different organic carbon contents were selected to evaluate the effects of ultraviolet radiation (UV-B，280 to 315 nm) on soil total carbon (TC), dissolved organic carbon (DOC), and alkali-extracted phenolics (AEP), and to characterize the responses of soils of different thicknesses (0.95 mm, 1.89 mm, 2.84 mm) to UV-B radiation by simulating incubation experiment. The results demonstrated that, compared with dark treatment, UV-B radiation significantly decreased soil TC by 3.11% and 6.18%, but increased DOC concentration by 16.05% and 9.89% for the two soils with low (A) or high (B) organic carbon content at the end of experiment. After 96 h UV-B radiation, compared to the soil of 1.89 mm and 2.84 mm thickness, UV-B radiation had more significant effects on soil TC, DOC and AEP in the thinnest soil (0.95 mm), TC was reduced by 19.11% and 14.35%, but DOC concentration was increased by 18.66% and 18.92%, and AEP concentration was increased by 23.52% and 22.70% in the thinnest treatments (0.95 mm) for soil A and B, respectively, but there was no significant difference observed between the soils of 1.89 mm and 2.84 mm thickness. It was concluded that UV-B radiation had significant effects on the stability of soil carbon pool, and the completely bare surface should be reduced as much as possible to protect the stability of the soil carbon pool.