Abstract: In order to investigate the performance of vertical cutoff wall for a simple dump landfill, a finite difference model is set up on the basis of the geological condition of the Qizishan landfill in Suzhou. The breakthrough time of vertical cutoff wall by the contaminant of COD is analyzed considering different materials and design parameters. The parametric study shows that cross hydraulic head, wall permeability, retarding factor and design thickness are the primary parameters to affect the breakthrough time of the cut off wall being keyed into an aquitard layer. The effect of geological condition on the keyed-in cut off wall is relatively insignificant. The influence of diffusion coefficient can not be neglected, especially for the condition of low cross hydraulic head.