Geosynthetic clay liners (GCL) composite vertical anti-seepage wall, which consists of a cement-bentonite slurry wall and geosynthetic clay liners, is a new type of vertical anti-seepage wall. GCL overlap width and the treatment effect of the overlap area are the important factors that influence the impervious performance of a GCL composite vertical barrier wall. A two-dimensional model for pollutant migration was presented in this study. The study conducted a comprehensive analysis of the impact of various factors, including the width of GCL overlap (B₂), the permeability coefficient of the GCL overlap area (k_p)_, the thickness of the cement-bentonite wall (L_w) and the permeability coefficient of the cement-bentonite wall (k_w), on the transversal distribution of pollutants and the critical GCL overlap width. This analysis was based on a numerical simulation method. The calculation results showed that the bypass effect induced by the GCL overlap area has a significant influence on the anti-seepage performance. Keeping the other parameters identical, when the k_p of the GCL overlap area is reduced from 1.0×10^-6 m/s to 1.0×10^-10 m/s, the critical GCL overlap width decreases considerably from 68.9 cm to 21.1 cm. Increasing L_w leads to a decrease in pollutant concentration at the exit of the overlap area and a reduction in the critical overlap width of the GCL. For example, when L_w is increased from 0.4 to 0.8 m, the breakthrough time is extended by approximately 2.5 times and the critical GCL overlap width is reduced by about 38.5%. Furthermore, when the k_w of the cement-bentonite wall is reduced from 1.0×10^-8 m/s to 5.0×10^-10 m/s, the concentration of pollutant at the exit of the overlap area is decreased by 73.1 % and the critical width of the GCL overlap is reduced by about 33.3%.