During in situ thermal injection in oil shale mining, the shear properties of oil shale at real-time high temperatures considerably affect the stability of injection and production wellbore as well as oil and gas production. The results of this study show that the shear strength and modulus of oil shale decrease with an increase in the shear angle at real-time high temperatures. With increasing temperature, the shear strength and modulus first decrease and then increase, reaching their lowest values at 400 °C. Thereafter, at temperatures above 400 °C, the energy accumulated in the elasticity and crack propagation stages is released and a large amount of energy is liberated during the instability failure. Finally, with rising temperature, the failure stage of oil shale changes from brittle to ductile, secondary fractures gradually increase and the failure characteristics step-by-step turn from through to nonthrough cracking characteristics.