Cutoff wall connected with composite geomembrane is commonly used as an impervious system in cofferdam projects. The geomembrane at the junction often suffers from structural damage and causes concentrated leakage. In line with the mechanical characteristics of composite geomembrane in the impervious system of high earth-rock cofferdam, we established the structural models of the connection between composite geomembrane and cutoff wall considering three connection forms: no expansion joint, ordinary vertical expansion joint, and U-shaped groove expansion joint using a self-developed large-scale bidirectional low friction laminar-ring simple shear apparatus. We further conducted large-scale shear test on the connection between composite geomembrane and cutoff wall to probe into the mechanical deformation mechanism of the composite geomembrane under different connection forms. Results unveiled that in the aforementioned three connection forms, 1) the strain of geomembrane gradually increased with the gradual increase of shear displacement while declined with the growth of distance from the cutoff wall; 2) the variation trend of stress-strain relationship of the model soil didn’t vary much with the connection form, and the peak stress saw no big change; 3) the peak strain of geomembrane near the cut-off wall with no expansion joint was 4×10^-3, with ordinary vertical expansion joints, 3.5×10^-3, and with U-shaped groove expansion joints, 6.51×10^-4. Compared with the other two forms, U-shaped groove expansion joints reduced the peak strain of geomembrane markedly and effectively solved the problem of strain concentration at the junction between geomembrane and cutoff wall.