Leather products have a long history of wide use in China, but due to their main components being proteins, lipids and other organic compounds, they are easily affected by burial conditions (soil, microorganisms and underground water, etc.) or museum environments (temperature, humidity, light, microorganisms and harmful gases, etc.) and thus suffer from a series of diseases (hardening, insect decay, rot, deformation, discoloration, damage and loss, etc.), among which hardening is the most common and serious. To explore the conservation methods for softening hardened leather relics, artificially-aged modern sheepskins were used as the samples to be softened by different formulae of softening agents. In order to compare the softening effects of different softening agents, the softness, mass and dimensional change data of leather samples before and after softening were tested, and the microstructures observed using scanning electron microscopy (SEM) were analyzed. The results show that the softening agents with the ratio of lanolin∶Span-80∶silicone oil∶water = 10∶1∶1.5∶50 and the ratio of lanolin∶SDBS∶silicone oil∶water = 10∶1∶1.5∶50 have obvious softening effects on the hardened leather samples, and could be widely used for the conservation of hardened leather cultural relics.