The chaotic attitude motion of a spacecraft with three rotors subjected to small external perturbation torques was investigated. By using the Deprit's canonical variables, the Hamilton structure of the system was established. Melniklv's method was carried out to predict the transversal intersections of the stable and unstable manifolds and to achieve the conditions of the occurring of chaotic motion. The study shows that the range of the excitation frequencies, which excite the occurring of chaotic dynamics, decreases as the momentum moment of the wheels increases. Finally, the numerical simulation of phase space shows the reliability of the theoretical analysis.