High-pressure anaerobic digestion (HPAD) can directly upgrade biogas (CH4 content to 90%) within the reactor. However, critical operational parameters, including hydraulic retention time (HRT) and pressure remained limited investigation. In this study, an HPAD reactor was operated at five different HRT (from 40 to 13 days), with pressure around 10~13 bar. In HPAD, the pressure was the driving force behind CH4 content. Low HRT (< 20 days) led to volatile fatty acid accumulation, which was earlier than the normal pressure counterpart. HRT had more effect on Archaea than Bacteria, while such a trend was inverted for pressure. Hydrogenotrophic methanogen Methanobacterium prevailed at low HRT (13~20 days). When operating continuous HPAD, attention should be paid to HRT, as low HRT (13 days) impaired the activity of CH4-synthesizing enzyme Methyl-coenzyme M reductase (MCR).