Abstract: As one of the key technologies for green hydrogen production, alkaline water electrolysis (AWE) has been widely adopted due to its low equipment cost and relatively mature technology. However, conventional electrolyzer designs still face significant challenges, such as low gas–liquid separation efficiency, high polarization losses, and uneven flow distribution, which limit the overall energy efficiency and operational stability of the system. To gain deeper insights into the coupled mechanisms of mass transfer, heat transfer, and electrochemical reactions within alkaline electrolyzers, and to enhance both system performance and structural design, this study focuses on electrolyzers with expanded mesh structures. A systematic investigation is conducted on key design parameters, including geometric factors (mesh opening slope length, minor axis length, and mesh height) and flow configurations (flow direction), using a multiphysics coupling approach. A three-dimensional multiphysics model is developed, incorporating two-phase gas–liquid flow, heat transfer, and electrochemical reaction processes, to elucidate the interactions among the complex physical phenomena occurring inside the cell. The numerical study consists of three main steps: electrochemical initialization, coupled simulation of the flow and thermal fields, and post-processing analysis. The simulation results reveal the influence of structural variables on bubble removal efficiency, local current density distribution, and system pressure drop. Specifically, counter-flow configurations enhance bubble detachment and promote disturbance in the reaction zone, thereby improving mass transfer efficiency and current uniformity. Optimizing the major axis length offers a favorable trade-off between pressure drop and performance improvement, making it a key factor in structural optimization. Additionally, tuning the mesh height requires a careful balance between inducing flow disturbance and maintaining smooth electrolyte transport. This work provides valuable insights and design guidelines for optimizing the flow and electrochemical performance of alkaline electrolyzers equipped with expanded mesh flow channels.