Permeability of mudstone interlayer is an important factor to evaluate the sealing property of salt cavern gas storage. The permeability of a certain mudstone interlayer in Jintan is relatively high according to pressure tests, and thus it is inappropriate for the sealing performance. Since the borehole cores of mudstone interlayer are easy to be broken, it is substantially difficult to carry out systematic permeability tests. To solve the problem, we proposed a new method for testing the permeability of brittle mudstone interlayer. The brittle interlayer cores tightly wrapped by epoxy resin were cut and polished to prepare standard samples for meeting the requirement of permeability tests. To observe the cement situation of the mudstone interlayer cores and the epoxy resin, the synthetic samples are carried out by scanning electron microscope (SEM). It is found that the pore of the interface is lower than that of rock and the interface cement as well, which shows that the interlayer core is the main seepage channel of synthetic samples. The results of permeability tests on synthetic samples by using a steady-state method are consistent with other results in the literature, which proves the reliability of this new method. The theoretical analysis also shows that the permeability results measured by the new method are consistent with the theory, and thus the feasibility of the method is verified theoretically. The new method of testing the permeability of mudstone interlayer proposed is feasible and reliable. This method has a certain reference value for the test of the permeability of the irregular brittle core.