As a critical vulnerability in the piping systems of energy facilities, the structural integrity of copper–nickel (Cu-Ni) alloy welded joints under microbial environments has garnered significant attention. This study investigates the divergent corrosion behaviors of 90/10 Cu-Ni alloy welded joints induced by sulfate-reducing bacteria (SRB) through simulated experiments, comparing bulk immersion and thin liquid film (TLF) environments. The results demonstrate that under identical SRB inoculation concentrations, the localized corrosion of welded joints is significantly intensified in the TLF environment. Notably, the average depth of intergranular corrosion (IGC) trenches in TLF is approximately 50% greater than that observed in bulk immersion. This acceleration effect is primarily attributed to the restricted diffusion of metabolic products within the confined liquid phase space of the TLF. Mechanistic analysis reveals that despite the strictly anaerobic conditions, the limited volume of the TLF leads to the drastic enrichment of SRB cells and their metabolic sulfides (HS⁻) at the weld metal (WM) and heat-affected zone (HAZ), triggering severe localized MIC. Electrochemical Impedance Spectroscopy (EIS) further confirms that the sulfide-based corrosion product films formed under TLF exhibit higher porosity and lower charge transfer resistance, failing to provide effective passivity. This work elucidates the MIC evolution patterns under the specific service condition of anaerobic thin liquid films, offering a novel perspective for risk identification and integrity management of critical energy infrastructure components.