When composites structures are impacted by an external low-velocity object, it is easy to cause the subsurface damage yet undetectable by visual inspection. These damages will reduce structural bearing capacity. In this paper, an experimental study was conducted on a cylindrical stiffened composite structure to investigate several key issues on impact load identification based on system identification method. Firstly, the selection principle of the model order of the transfer function was discussed and clarified to improve the performance. Furthermore, to overcome the limitation that the homogeneity of sensor responses is not ensured due to the manufacturing and installation process of piezoelectric sensors, a sensor signal correction method was proposed in order to improve the accuracy of force identification. At last, the influence of stiffeners on the impact identification techniques was analyzed and discussed. The experimental results indicated that the method can be feasible for improving the identification and reconstruction of external discrete impact force on cylindrical stiffened composite structures.