Exploration on the transient evolution of the rheological properties of dense granular inertial flow is essential for revealing how the balance is established between the boundary drive strength and the internal shear strength. In this paper, discrete element method simulations are performed to study the transient flow characteristics of a dense granular system under plane shear in the inertial regime. To this end, we quantitatively analyze the changes in the system’s flow state, interfacial friction coefficient, internal friction coefficient, and microstructure. Simulation results show that the evolution of the horizontal flow experiences three typical stages, namely transmission, adjustment, and stabilization. Meanwhile, the shear dilatancy caused by the vertical movement of particles, gradually loosens the filling state, weakens the spatial geometric constraint and the system’s tangential load-bearing capacity, thereby decreasing the interfacial friction coefficient and reducing the boundary drive strength. On the other hand, the variations in the anisotropies of both contact orientation and contact forces, increase the internal friction coefficient and improve the internal shear strength. Therefore, the evolution of flow state from initially static to finally stable reduces the boundary drive strength while enhances the internal shear strength, and eventually a balance between them is achieved. Distinguished from the micromechanical behaviors, under different shear velocities the internal shear strength always mainly originates from the anisotropies in contact orientation and in normal contact force. Moreover, the contribution of the anisotropy in contact orientation becomes more predominant with the increase of shear velocity.