The numerical simulation way studied hypervelocity impact has been discussed. Damage mechanism and the process of the debris cloud forming has been studied with the SPH method of ANSYS/AUTODYN code when spherical projectile hypervelocity impact thin shield. Tungsten alloy and RHA(Rolled Homogeneous Armor) material models and parameters have been given, using the methods, material models and parameters of the numerical simulation results in good agreement with the experimental results. fragmentation-initiation threshold velocity of the projectile, the average fragment size and the distributing of fragment size have been researched including its relation with the impacting velocities when the projectile hypervelocity impact the target.