To investigate the effect of joint inclination on the mechanical behaviors of unloading-induced slip processes, shale samples with different joint inclinations (θ) were tested under constant axial stress and graded unloading of confining pressure to induce slip. The results show that the slip tests resulted in scratches and damage of the rock matrix, thus increasing joint roughness coefficient (JRC) of saw-cut surface samples by values of 4.69, 2.75 and 2.67 for θ = 30°, 40° and 50°, respectively. With the unloading of confining pressure, the samples experienced creep slip, quasi-static slip, and dynamic slip, respectively. The velocity at creep slip, quasi-static slip and dynamic slip decreased with the increase in θ. The average velocities of quasi-static slip are 2.9 × 10−2 mm/s, 1.1 × 10−2 mm/s and 2.1 × 10−3 mm/s for θ = 30°, 40° and 50°, respectively. As the slip progresses, the micro-convexities on the joint are sheared off, resulting in a deterioration of the interlocking effect, which causes the occurrence time of quasi-static slip to advance in an unloading stage and the duration to increase. The friction coefficient (μ) decreased by 2.22%, which changes from 0.45 to 0.44 for the sample with θ = 30°, indicating that the friction strength was slightly weakened during the unloading-induced slip. As θ increases, the friction strength of the sample was enhanced. The μ increased by 19.23% from 0.52 to 0.62 and increased by 28.81% from 0.59 to 0.76 for the sample with θ=40° and 50°, respectively. Based on the experimental results, the unloading-induced slip behaviors were fitted with the rate and state friction law. However, the mechanical behavior of the sample has changed from “shear slip” to “slip failure” as the joint inclination increase, which resulted in the dynamic slip of the sample with θ = 40° and 50° being opposite to the rate weakening described in the rate state friction law.