This study investigates the corrosion status of the gathering and transportation system in the Jianghan Oilfield—the first domestic H₂S-bearing shale gas block—using L245NS and L360NS pipeline steels. By employing the extinct-dilution method, laboratory corrosion experiments, and analyses through SEM, EDS, and XRD, the study elucidates the corrosion mechanism under a harsh environment characterized by moderate H₂S, CO₂, and high microbial content. Results indicate that water samples contained significant populations of sulfate-reducing bacteria (SRB, 5 × 105 cells/mL), thiogenic bacteria (TGB, 2 × 105 cells/mL), and iron-oxidizing bacteria (FB, 1.3 × 105 cells/mL). Corrosion was driven synergistically by electrochemical processes induced by “CO₂ + H₂S” and microbiologically influenced corrosion. The corrosion products primarily consisted of loose, porous FeCO₃ and FeS, which lost barrier efficacy and intensified localized attacks. The average corrosion rate of L360NS steel was 0.1699 mm/a, slightly lower than that of L245NS (0.1770 mm/a), although both steels underwent severe corrosion. The recommended optimal concentration for adding CT2–19 imidazoline corrosion inhibitor to the Hongxing block is 0.5%. These findings provide technical support for the design and implementation of corrosion control measures in the large-scale development and production of this shale gas block.