Most of the existing unconventional oil and gas reservoirs in China are tested with cylindrical samples by the false triaxial test device. However, under the condition of true triaxial, the edges of cube specimens are mostly deficient in edge flow and face seal. Based on the existing true triaxial test device and a large number of experiments, the existing seepage sealing method of cube specimen with single rubber sleeve was improved. This paper introduces the structural features of nested sealing sleeve, especially the design of the seal in the edge area, puts forward the structural design of the top and bottom end of the sealing sleeve concave with a single hole mesh groove, and discusses the size of the top pressure transfer plate. Taking the 100 mm×100 mm×100 mm cubic coal sample as an example, the true triaxial seepage test analysis was carried out, and the permeability under different inlet seepage pressures under true triaxial confining pressures was obtained. The improvement of the sealing methodddd is helpful to improve the permeability test accuracy of unconventional oil and gas reservoirs under in-situ stress conditions in scientific research institutions and testing centers, and improve the practical innovation ability and scientific research accomplishment of high-end scientific research talents.