Abstract: The mass transfer in catalyst capsule is analysed.It is pointed out that the wetting incompletion of catalyst beads and the concentration gradient within the catalyst capsule reduce catalyst efficiency.The effectiveness factor of catalyst capsule equals to the product of η _s and η _c.The former,the effective wetting rate of catalyst beads,represents the influences of wetting incompletion of catalyst on catalyst efficiency,the latter represents that of concentration gradient.A two- dimensional model is established and an equation to calculate η _c is given.It is indicated that η _c is influenced not only by radial diffusion,but also by axial liquid flow.The influence of diffusion resistance on effectiveness factor is greatly weakened by axial liquid flow.