The problem of attitude regulation control of an underactuated spacecraft is resolved by using sliding mode method. The three-axis underactuated spacecraft attitude dynamics and kinematics models are introduced. A sliding mode controller using generalized inverse and second order approaching law is designed for underactuated axis stabilization control, and the realisability of the controller is analyzed. A perturbed null-projection is constructed to guarantee the feasibility of the controller. On the basis of stabilization of the underactuated axis, a sliding mode controller is designed for another two axes as well. The proof of bounded stability is given in Lyapunov's sense. Simulation results demonstrate the availability of the proposed control algorithm. 