The invention discloses a device for testing the penetration and deformation of an impervious material of a gasket of a landfill site. The device comprises a pressurization device, a data control device, a testing vessel and a data processing device; the testing vessel consists of two hollow hemispheres with wing edges; the impervious material of the gasket is arranged between two hollow hemispheres; and the testing device measures pressure at two sides of geomembrane to obtain the pressure difference, provides parameters for the deformation of the geomembrane under certain pressure and can establish a pressure-deformation quantity relation. The testing device has the functions of measuring the osmotic coefficient and the deformation of the impervious material of the landfill site and also has the function of measuring the osmotic coefficients under different pressures. Data acquisition of the testing device ensures the simultaneity of the data acquisition to allow the measured osmotic coefficient to be more accurate. The osmotic coefficient measured by the testing device can be directly read and also establish relations between the pressure and the osmotic coefficient and betweenthe pressure and the deformation quantity; therefore, the device provides more reliable evidence for the engineering application and the estimation of imperviousness of the geomembrane.