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[Display omitted] • The effect of acetate to butyrate ratio in electron acceptor (EA) on CCE was studied. • The higher content of butyrate in feedstock led to the higher production of caproate. • The maximum production of caproate was 3.7 ± 0.3 g·L−1, when butyrate was the only EA. • Butyrate would be selected as EA only when acetate was much lower than butyrate. • Metagenomic analysis proved that high butyrate content strengthened the CCE process. This study investigated the effect of electron acceptor (EA) distribution (acetate to butyrate ratio) on the carbon chain elongation (CCE) process. The results showed that the higher content of butyrate in the initial material led to the higher production of caproate. The maximum production of caproate was 3.74 ± 0.30 g·L−1, which was obtained when only butyrate was added as EA. Little caproate but much butyrate was produced where only acetate was added as EA. This indicated that CCE bacteria preferentially selected acetate as the EA to produce butyrate, and butyrate could be selected as EA to produce caproate only when the acetate content was much lower than butyrate. Unclassified_f_Dysgonomonadaceae , Massilibacterium , and Seramator were the predominant bacteria. Functional enzyme analysis showed that high butyrate content strengthened the fatty acid biosynthesis pathway and reverse β-oxidization pathway. The findings showed the importance of butyrate in CCE for caproate production. [ABSTRACT FROM AUTHOR]