The shock characteristics of a cubic nonlinear packaging system with critical component were investigated under action of rectangular acceleration pulse. The dynamic model of the system was developed, and the numerical results of the dynamic equations were gained. Based on the results, the three-dimensional shock response spectra of the critical component was obtained. The effect of pulse duration, frequency ratio, dmaping ratio, pulse peak acceleration and the system parameters on the maximum shock response acceleration of the critical component was discussed.It was shown that all the effects are noticeable.The results led to some insights into the design of cushioning packaging.