<p>High-load anaerobic digestion can enhance both economic benefits and energy efficiency, but stable operation remains challenging. The study employed a modified continuous stirred tank reactor (CSTR) to investigate operational stability and metabolic shift during long-term co-digestion of food waste and chicken manure under stepwise increasing organic loading rates (OLRs) from 2 to 8&#xa0;g-VS·L^− 1·d^− 1 (VS refer to volatile solid). The system could be stably operated at OLRs up to 7&#xa0;g-VS·L^− 1·d^− 1, reaching the highest energy yield of 14.0 kJ·g^− 1-VS. However, instability occurred at 8&#xa0;g-VS·L^− 1·d^− 1, accompanied by volatile fatty acids (VFAs) accumulation and pH decline. The biogas and methane production were positively correlated to the OLR at stable period. High VS removal efficiency of 87.05%-91.13% can be ascribed to the sludge settling within the modified CSTR reactor. The sludge recirculation played significant roles in promoting methane yield and system stability at 6–7&#xa0;g-VS·L^− 1·d^− 1 through the effects of reducing VFAs, raising pH value and adjusting microbial community. The acetoclastic <i>Methanosaeta</i> was predominant in stable period. Without sludge recirculation at 5&#xa0;g-VS·L^− 1·d^− 1, the metabolic pathway of syntrophic acetate oxidation (SAO) combined with hydrogenotrophic methanogenesis (HM) was enhanced by the high VFAs. However, acetoclastic methanogens <i>Methanosaeta</i> predominated again when the bottom sludge was recirculated at 6–7&#xa0;g-VS·L^− 1·d^− 1. With the extremely high VFAs content at OLR of 8&#xa0;g-VS·L^− 1·d^− 1, the SAO-HM pathway regained dominance to overcome VFAs accumulation.</p> Graphical Abstract <p></p>