Effects of treating early rice straw returning to field with two types of decomposition accelerant, Fuganling and straw-decomposition microbe, on yield formation of late rice were studied in double cropping rice field in Changsha, Hunan. The results showed the decomposition accelerant treated early rice straw had significant promoting effect on rice yield by promoting tillering and differentiation of spike and increasing leaf area thus enhancing effective-panicle number, grain number per panicle and ratio of grain number to leaf area and dry matter accumulation correspondingly, and the yield of late rice increased 8%-12%. Compared to Fuganling treatment, effective panicle number, grain number per panicle, leaf area index (LAI) at booting stage, yield and output percent of dry matter in stem and sheath (OPDS) in treatment applied straw-decomposition microbe were increased by 1.7%, 1.5%, 4.3%, 3.9% and 17.45% respectively, and the differences in LAI at booting stage and OPDS between the two treatments were significant.