A two-phase power reaching law-based attitude control scheme is proposed for spacecrafts with system uncertainties. In the presented reaching law design, the power value is adjusted according to the change of sliding mode variable value, such that the fast convergence rate of the sliding mode variable can be achieved when it is far away from and approaching to the equilibrium. Meanwhile, a relatively accurate convergence time expression of the sliding mode variable is obtained directly by calculating two convergence time in different reaching stages, respectively. A robust term is adopted in the control design to compensate for the effect of uncertainties and disturbances, and thus the system robustness is enhanced. Finally, the stability of the system is analyzed based on the Lyapunov stability synthesis, and the effectiveness of the proposed control scheme is verified by comparative simulations.