Microbial treatment of crop straws is an important approach to feed utilization of straws, which is of great significance for realizing efficient and safe utilization of straws and promoting the production of herbivorous livestock. To evaluate the effects of two kinds of Bacillus on degradation and microsilage of straw fiber, a 45-day fermentation experiment was performed on corn, rice and wheat straws treated with Bacillus subtilis and Bacillus velezensis and their combinations. The results showed that Bacillus velezensis had a significant effect on the degradation of the three fibers, Bacillus velezensis treatment reduced neutral detergent fiber (NDF), acid detergent fiber (ADF) and acid detergent lignin (ADL) by 7.31%, 12.0% and 21.4% for corn straw, and 9.25%, 16.0% and 43.0% for rice straw, respectively. ADF of wheat straw by 6.52%. The Bacillus subtilis treatments reduced NDF, ADF and ADL of corn straw by 7.19%, 13.1% and 24.8%, respectively. ADF of wheat straw by 6.96%, but it had no significant effect on rice straw, and the effect of mixed treatment of the two Bacillus species was not as significant as the single Bacillus specie treatment. The fermentation products of straw treated with both Bacillus species were mainly acetic acid and butyric acid, and lactic acid was not detected. Overall, both Bacillus velezensis and Bacillus subtilis could significantly reduce the NDF and ADF contents of corn and wheat straw, but the presence of butyric acid in the fermentation products decreased the fermentation quality. Therefore, in order to enhance the quality of micro-silage of straw, the complex microbial inoculants effect of both Bacillus species with other strains needs to be explored in the future.