Ethanol and lactate as co-electron donors (co-EDs) on chain elongation (CE) have been studied, however, the effects of ethanol on lactate-driven CE remain unknown. This study evaluated the caproate production performance from lactate under different ethanol additions (7.67-23.01 g/L), and determined the optimal ethanol concentration. The results showed that the highest caproate yield of 4.51 g/L was obtained at 15.34 g/L ethanol concentration. The synergistic effects of co-EDs on caproate production were enhanced due to more stable pH and higher reducing power under appropriate ethanol stimulation. Functional bacteria related to caproate production, including Sporanaerobacter and Clostridium_senseu_stricto_12, were specifically enriched under appropriate ethanol stimulation. The ethanol toxicity caused by excessive concentration (more than 23.01 g/L) inhibited the growth of key CE bacteria and reduced the production of caproate. This study revealed the dual effects of ethanol in the co-EDs fermentation system for caproate production and determined the threshold of ethanol addition.