The development of photoactive materials based on single-atom catalysts is essential to improve the performance of photoelectrochemical (PEC) biosensors. Herein, we have designed Pt atoms-anchored MoS₂ nanotubes (Pt-MoS₂NT) as photoactive materials. Catalytic kinetics of Pt-MoS₂NT can enable efficient separation of photogenerated electron-hole pairs and extend the charge carrier lifetime, which makes Pt-MoS₂NT possess a significantly enhanced photocurrent response compared to MoS₂NT. During the detection of miRNA-155, horseradish peroxidase-catalyzed reaction produces a solid precipitate (benzo-4-chlorohexadienone), which can block the electron transport pathway between the electrolyte solution and Pt-MoS₂NT, resulting in a significant reduction of the PEC signal. Based on a dual strategy of single-atoms Pt enhanced photocurrent response and enzymatic reaction, the constructed PEC biosensor achieves ultra-sensitive detection of miRNA-155 (detection limit up to 3.1 aM). The present work confirms the advantages of single-atom catalysts for bioassay applications.