One-dimensional strain impact experiments were performed for the high purity aluminum (HPA) (99.999%). The spall characteristics of HPA in dynamic tensile fracture have been discussed according to the quantitative metallographic analysis of the shock recover of samples. By defining the product of the tensile stress and the time as a parameter called tensile impulse (I), the statistic resultsindicate that an obvious critical behavior for the damage evolution appears with the increasing of I. When the I is low, the damage grows slowly with alinear increment. Once the I reaches a critical value, the damage grows rapidly and an exponential increase is observed. Our preliminary results indicate that the critical value of I for HPA is about 0.34 GPa·μs and the corresponding critical damage is about 0.12.