In the study, starch-rich sorghum was used as raw material to improve the production efficiency of caproic acid by two-step anaerobic fermentation. The first step was to inoculate caproic acid compound bacteria, inoculate Lactobacillus acidophilus TYCA06, and add Saccharomyces cerevisiae to produce butyric acid fermentation broth, lactic acid fermentation broth and ethanol fermentation broth. The second step was to mix butyric acid fermentation broth, lactic acid fermentation broth and ethanol fermentation broth, and then inoculate caproic acid compound bacteria to produce caproic acid by anaerobic fermentation. The results showed that by optimizing the fermentation process of butyric acid fermentation broth, lactic acid fermentation broth and ethanol fermentation broth, the butyric acid and acetic acid yield in butyric acid fermentation broth reached 20.16g/L and 8.47g/L, and the starch conversion rate reached 76.02%. The lactic acid yield and lactic acid yield rate in lactic acid fermentation broth reached 11.41g/L and 42.48% respectively. The ethanol yield in ethanol fermentation broth reached 95.06%. Continuing to optimize the caproic acid fermentation process, it was found that when ethanol fermentation broth and lactic acid fermentation broth were used as electron donors at the same time, the yield and yield rate of caproic acid were significantly increased, reaching 4.15g/L and 75.2mg/g, respectively, and when the mass concentration ratio of ethanol to lactic acid added in ethanol fermentation broth and lactic acid fermentation broth was 2∶1, the yield and yield rate of caproic acid reached the highest, reaching 6.65g/L and 99.78mg/g, respectively. According to the correlation analysis between microorganisms and caproic acid in the process of caproic acid fermentation, Caproiciproducens, Clostridium_sensu_stricto_12 and Pseudoclavibacter were positively correlated with caproic acid content in the fermentation process. Among them, Caproiciproducens was continuously enriched with fermentation, and the relative abundance reached 37.81% in the later stage of fermentation, which was the main contributor to the formation of caproic acid. It was hoped that the study could provide a theoretical basis for the production of caproic acid by two-step anaerobic fermentation of sorghum without exogenous electron donor, and provide a new idea for expanding the resource utilization of sorghum.