Abstract: The composite geomembrane cut-off wall (CGCW) is one of the most effective technologies to prevent the horizontal migration of contaminants at present. Considering the characteristics of groundwater seepage in the outside aquifer, a numerical model for migration of the organic contaminants into the CGCW-aquifer system is established, and is solved using the finite element software COMSOL 5.3. The influences of parameters of the aquifer and CGCW are comprehensively investigated. The increase of the groundwater flow rate in the outside aquifer will accelerate the migration of contaminants and decrease the concentration of contaminants in the CGCW. In addition, when the seepage velocity is higher than 1×10^-5 m/s and lower than 1×10^-9 m/s, the models based on the boundary conditions of zero concentration and non-advection aquifer can be applied to the preliminary design of CGCW. Furthermore, the optimal location of the geomembrane in the CGCW is closely related to the type of contaminants and groundwater flow rate of the outsider aquifer, and the difference among the cumulative mass fluxes of the outlet face in the CGCW at different geomembrane locations can reach 10%~20%. The performance of the CGCW with EVOH geomembrane is significantly better than that of the CGCW with HDPE geomembrane.