Diffusion is an important phenomenon that governs contaminant transport through compacted clay liner at the bottom of landfill. The diffusion process is influenced by many factors, i.e., type, valence and concentration of ions. Therefore, an accurate diffusion model is of great importance for the design of liner system of landfill. The simplified Guntelberg activity coefficient was introduced to indicate concentration difference between ideal solution and actual solution. The driving force of diffusion usually considered combined action of chemical and diffusion potential. The semipermeable membrane effect of clay was considered. A diffusion model describing simultaneous movement of multi-ions through clay barrier was proposed. Moreover, the multi-physical field simulation software COMSOL Multiphysics was employed for conducting simulations for the proposed diffusion model. The results indicate that the factors such as solution non-ideality, diffusion potential, and semipermeable membrane effect of clay play an important role in the diffusion process of multiple ions. A remarkable retarded behavior was captured, when the solution non-ideality or semipermeable membrane effect was considered in the proposed model. Further, an accelerating effect on cations was observed taking diffusion potential into consideration. The multi-ion diffusion process under combined action is not a simple superposition of single influence mechanism, but rather, each mechanism restricts each other and affects the multi-ion diffusion behavior.