Abstract: The cathode catalytic layer serves as the critical site for oxygen reduction reactions within proton exchange membrane fuel cells, and optimising its carbon carrier structure is paramount for enhancing fuel cell performance. A Shan-Chen pseudopotential lattice Boltzmann method is employed to establish a coupled two-phase flow model that integrates oxygen reduction reaction and mass transport. It investigates the influence of mesoporous carbon carrier structures—featuring interconnected pores and gradient pores—on both the oxygen reduction reaction and transport processes within the cathode catalytic layer of proton exchange membrane fuel cells. Results indicate that, compared to non-interconnected pore structures, interconnected mesoporous carbon structures provide ample oxygen transport pathways and drainage routes. For mesoporous carbon catalyst layers with gradient pores (outer depth 6 nm, inner depth 8 nm), the oxygen reduction reaction rate initially increases, then decreases with increasing outer pore depth, reaching a maximum at an outer depth of 3 nm. In contrast, for mesoporous carbon carriers with gradient pores (outer 8 nm, inner 6 nm), the oxygen reduction reaction rate decreased with increasing polymer intrusion ratio. Therefore, designing mesoporous carbon carriers with interconnected pores and employing a gradient pore structure (outer small, inner large, with an outer pore depth of approximately 3 nm) can optimise oxygen transport and liquid water drainage within the catalytic layer, effectively enhancing the performance of proton exchange membrane fuel cells.