In the autonomous navigation state, the periodic errors, such as the Schuler oscillation amplitude error, in the strap-down attitude measurement system(SAMS) can be restrained through the internal horizontal damping method. However, the schuler tuning condition of the SAMS will be destroyed when the damping network was introduced, and new attitude errors will be generated if the motion of the carrier changes quickly. In order to overcome the shortcomings of the conventional damping network with fixed damping ratio, a novel damping network with continuously adjustable damping ratio was designed to suppress the attitude error. The relationship between the acceleration of the carrier, the damping ratio and the attitude error was also derived. An adaptive-damping method was proposed to adjust the damping ratio of the system according to the carrier′s acceleration. The simulation results and experiment results indicate that the novel damping network can suppress the Schuler oscillation amplitude error, and the adaptive-damping method can reduce the ill effect of the damping network to the attitude estimation, and improve the performance of the SAMS.