The invention relates to a large-scale real triaxial shear testing device and an application thereof. The testing device comprises a testing box, a hydraulic pressurization mechanism and a water pressure pressurization mechanism, wherein the testing box comprises a box frame, a shear box and a soil pocket; the hydraulic pressurization mechanism comprises a hydraulic control box, a vertical hydraulic cylinder, a horizontal hydraulic cylinder and a shear testing hydraulic cylinder; the water pressure pressurization mechanism comprises a water box, a gas pressure tank and a water pressure pocket; the hydraulic control box is used for controlling the pressure of the vertical hydraulic cylinder and the horizontal hydraulic cylinder, the gas pressure tank is used for controlling the pressure of the water pressure pocket and enabling a test sample in the soil pocket to be in a three-dimensional stress state, shear testing is performed on the test sample through the shear testing hydraulic cylinder, whether the test sample has drainage or not is controlled through a drainage valve, and the changes in displacement over time and the changes of pressure applied in each direction over the time are recorded when the test sample is sheared. Compared with the prior art, the large-scale real triaxial shear testing device provided by the invention has the characteristics that the size of the test sample is large, a shear surface is not a traditional fixed horizontal plane, the large-scale real triaxial shear testing device can be used for shear testing of soil in a three-dimensional stress state, and parameters of a soil layer can be determined more accurately.