Abstract: The structure and size of carbon support play an important role in presenting the catalytic activity and stability of the catalyst. Mesoporous carbon(MPC) is selected as the carbon support and the influence of particle size of the mesoporous carbon is optimized and explored. Also, the enhanced mechanism of the catalytic activity and stability of the supported catalyst and the performance of the catalyst-based MEA are investigated. Four different particle sizes of MPC are studied, it is found that when the mesoporous carbon particle size is 1.7 μm, the loaded Pt₃Co alloy nanoparticles are well-dispersed, the activity of the catalyst and the adaptability of the ink are the best. The initial catalytic activity of the Pt₃Co/MPC-1.7 catalyst reached 537.1 mA/mg@0.9 V, and the single-cell performance of the catalyst with the optimal IC(1.0) reached 0.7 V@2000 mA/cm², which is significantly higher than that of Pt₃Co/EC300J catalyst. Accelerated stress test displayed that the mass specific activity of the catalyst based on mesoporous carbon Pt₃Co/MPC-1.7 only decreased by 23.4% after 30000 cycles, which is significantly better than that of Pt₃Co/EC300J(53%) due to the mesoporous carbon rich mesoporous structure which plays an important role in anchoring Pt₃Co nanoparticles, inhibiting the growth of small particles, and hindering the movement and coarsening of alloy particles.