The damage evolution law of loading gas-bearing coal and rock from yield deformation to instability failure is related to the loading stress path, and has a certain influence on the occurrence of coal and gas outburst. Based on the energy conservation theory, the expression of damage factor in the whole process of gas-bearing coal and rock from loading to instability failure is analyzed and deduced, and the evolution law of damage factor in the whole stress-strain process of gas-bearing coal and rock under conventional triaxial loading and variable-speed piecewise triaxial loading stress path is calculated and analyzed. The results show that under different loading stress paths, in the linear elastic stage, the damage factor of loaded gas-bearing coal and rock is equal to 0, the damage factor begins to increase rapidly in the plastic deformation stage, and the damage factor of coal and rock increases sharply in the stress drop stage. After entering the residual strength stage, the growth rate of damage factor of coal and rock slows down significantly. The increase of gas pressure can promote the damage development of coal and rock, resulting in the overall increase of the critical damage factor at the moment of instability and failure and the damage factor entering residual strength. The increase of confining pressure can inhibit the damage development of coal and rock, resulting in the overall decrease of the critical damage factor at the moment of instability and failure and the damage factor entering residual strength. Compared with conventional triaxial loading, the critical damage factor at the moment of instability and failure of gas-bearing coal and rock and the damage factor when entering the residual strength under variable-speed piecewise triaxial loading condition are larger as a whole, the shear slip surface of coal and rock specimen after failure is more obvious as a whole, the fracture connectivity is better as a whole, the fracture opening is also larger, and the broken coal and rock are easier to separate.