Abstract: Based on twin shear strength theory and some true triaxial strength testing data of rock, a more reasonable twin shear strength criterion is proposed and verified. The corresponding parameters of the twin shear strength criterion are given by traditional triaxial compressive and triaxial tensile testing results. It is shown that the twin shear strength criterion has a τ₈=g(θ_σ)·f(σ_m) function expression, and the calculating results in this paper show that τ_8c/τ_8t will be changed as the average principal stress σ_m is changed. τ_8c/τ_8t is larger when σ_m is at lower stress level, and is smaller when σ_m is at higher stress level. This phenomenon demonstrates on one side that rock is developed into homogeneous material gradually as σ_m is increased, on the other side that the Drucker-Prager strength criterion is not suitable for rock materials when σ_m is at lower stress level.