Corrugated steel liner reinforcement for existing tunnels exhibits characteristics such as high efficiency and durability. However, the grout diffusion mechanism behind corrugated steel liners remains unclear. To address this, this study derives a slurry pressure formula for circumferential diffusion behind corrugated steel plates, investigates slurry pressure distribution during backfill grouting using Fluent numerical modeling, and conducts parametric analyses. Research results indicate: The deviation between numerical simulation results and theoretical predictions remains within 10%, with theoretical results being conservative. The circumferential slurry pressure demonstrates a progressive increase from the vault to the invert of the tunnel. This pressure distribution is governed by the combined effects of slurry self-weight and viscous shear forces. Grouting pressure positively correlates with slurry pressure, and slurry pressure exhibits higher sensitivity to grouting pressure variations under equal vault-invert pressure difference conditions. Corrugation dimensions directly affect longitudinal pressure distribution. For large waveforms (400 × 150 mm), grouting pressure must be increased by 5–10% to ensure filling compactness. These findings provide theoretical support for parameter selection in corrugated steel liner grouting.