In view of the serious nonlinearity, the multivariable coupling and parameter uncertainty in the nonlinear dynamic system of rotating rocket projectiles, an Improved Nonlinear Disturbance Observer (INDO) is proposed to observe the approximating feature of the disturbance of high-frequency motion.A backstepping sliding mode controller is designed by combining with the backstepping sliding mode control.And the stability of the system is proved. Finally, the performance of the Traditional Nonlinear Disturbance Observer (INDO) is compared with that of the improved observer.The simulation results show that the improved observer has higher estimation accuracy and stronger robustness.