Numerical models that accurately describe the nano/microscale transport phenomena in the proton exchange membrane fuel cells (PEMFC) are highly required. In this study, a multiscale PEMFC model is developed by coupling a pore-scale model for the catalyst layer (CL) and a cell-scale model. In the pore-scale model, oxygen transport and electrochemical reaction under different platinum loadings within porous agglomerates are simulated. The pore-scale model, as a sub-grid model, is then coupled into a two-phase and non-isothermal cell-scale model. The multiscale model developed successfully predicts the impact of Platinum loading on the limiting current density, while the traditional one with the classical agglomerate model fails to. Detail distributions of important variables including concentration, current density, and saturation are discussed. Furthermore, using the multiscale model, ionomer content is optimized to improve the cell performance as well as reduce the performance loss when Pt loading is reduced. It is demonstrated that multiscale model incorporating nano/microscopic transport phenomena is promising to more accurately reveal the coupling mechanisms of multiple reactive transport processes in PEMFCs.