Abstract: Effect of numerical fractures on the computational accuracy is analyzed by introducing artificial numerical fractures. A simple adding-particle technique, which can be easily applied in large deformations, is introduced to prevent numerical fractures. During the adding-particle process, the mass conservation and momentum conservation of the system are kept except that the energy has a little change. The experiments of steel projectile impacting on the airplane skin are simulated by the initial and improved SPH methods. The results show that numerical fractures can lead to very large computational errors. Compared with the initial SPH method, the improved SPH method can prevent numerical fractures effectively. The deformation diameter, the deformation depth and the residual velocity of the projectile obtained by the improved SPH method agree well with the experimental results.