Gaomiaozi (GMZ) bentonite has been identified as a potential buffer/backfill material for high-level radioactive nuclear waste (HLRW) repository in China. The mechanical behavior of buffer and backfill materials plays a significant role in the engineered barrier system of the HLRW repository. Beishan area is identified as the best candidate area of the national repository for HLRW, where ground water contains various dissolved chemical elements. Therefore, the effect of salt concentration on strength and microstructure of GMZ bentonite and its mixture with different sand ratios (0${\%}$, 30${\%}$ and 50${\%}$) was studied by performing a series of direct shear tests and scanning electron microscopy (SEM) examination. The results show that the stress-strain curve of the mixtures has noticeable strain-hardening, and the shear strength of pure bentonite is greater than that of the mixtures in pure water. The test results also show that shear strength of GMZ bentonite and its mixtures are noticeably improved with the increase of salt solution concentration. The friction angle is clearly increased with the salt solution concentration, but the cohesion intercept is not. The shear mechanism of the mixture can be explained by microstructure changes and the concept of effective bentonite density of specimens with different salt concentrations.