To minimize system uncertainty and external disturbance in attitude tracking control for rigid spacecraft， a predefined-time sliding mode controller （PTSMC） is proposed. First， the spacecraft attitude tracking system is developed with quaternion parameterization， and the predefined time sliding surface is designed using an error quaternion and error angular velocity. Then， considering the uncertainties and external disturbances of the spacecraft system， a PTSMC with a non conservative upper bound is designed， and the noise of the system is reduced using boundary layer technology. Finally， by designing the Lyapunov function， the predefined-time stability of the proposed controller and the non conservative upper bound of the system convergence are demonstrated. The simulation results show that using the proposed approach， the attitude tracking accuracy of rigid spacecraft can reach 1.5×10^-6 rad， and the angular velocity tracking accuracy can reach 2×10^-6 rad/s. Compared with the existing predefined time control and non singular terminal sliding mode control， the upper bound of the stabilization time of the proposed control is more non conservative and has higher tracking accuracy and robustness. The effectiveness of the control scheme is further illustrated by the attitude tracking experiment of the 3 DOF airborne platform. The angle tracking error is less than 0.1 rad， and the position tracking error is less than 0.2 m.