In order to clarify the effects of three different rice straw returning modes (rotted rice straw, direct return of rice straw, rice straw-derived biochar) on organic carbon activity in the coastal saline paddy soil, an indoor incubation experiment (at 25℃ and 270 days) was designed and conducted to study the potential effect of their equivalent carbon application on accumulation and stability of soil organic carbon, the contents of microbial biomass carbon and dissolved organic carbon. Results showed that application of rice straw-derived biochar increased significantly the accumulation and stability of soil organic carbon, and improved the content of microbial biomass carbon. However, it had no effect on the content of dissolved organic carbon. The rotted rice straw and direct return of rice straw treatments increased significantly the contents of soil dissolved organic carbon and microbial biomass at the beginning of incubation experiment, while they had no effects on the accumulation of carbon. Application of rotted rice straw decreased the stability of organic carbon. In conclusion, application of rice straw-derived biochar could increase the accumulation of organic carbon in soil, but it might also decrease activity of soil organic matter after a long-term application without any application of other organic materials. However, its application combined application with rotted rice straw or direct return of rice straw can reduce this negative effect.