For the most proper solution of annual increase of solid and other waste treatment in the limited landfill site, the global leading countries develop and keep expanding operation of the offshore waste landfill sites. In this report, we examined the most proper mix ratio of the water cut-off materials for the prevention and enforcement of the performance of the existing cut-off wall inter-connection parts of the offshore waste landfill site, which might be deteriorated by the external force inherent in the sea area. For acquiring the most proper mixed ratios in back-up water cut-off materials, we examined and evaluated the physical characteristics of the applied back-up water cut-off materials - on-site marine clay materials, which might be possible to be acquired near to site, admixtures and bentonite. Also, we applied those back-up water cut-off materials into the new concept applied to the vertical cut-off wall connection parts and investigated variation of the water cut-off performances throughout the changes of the permeability coefficient before and after deformation of the inter-connection joints of the vertical cut-off walls. For the investigation of the permeability performances, we carried out half sized mock-up tests in the laboratory and checked the variation of permeability coefficients referred to the changes in the applying location of back-up water cut-off materials and finally reach to application of back-up water cut-off materials as a proper solution of water cut-off performances in the inter-connection parts of the vertical cut-off wall at offshore waste landfill site.