Head actuator arm assembly (HAA) is the most important mechanical component of a mobile hard disk drive (HDD) and its shock dynamic response is a principal index of vibration resistance. In this paper, a finite element (FE) model is firstly developed in ANSYS of 2.5 inch (1 inch=25.4 mm) mobile hard disk. This model includes actuator arm, voice coil motor (VCM) and pivot bearing. The various step modal of HAA is calculated by FE model. Then the actuator arm vibration behavior is simulated with LSDYNA procedure. The influence of pulse waveform, pulse amplitude and pulse width on the shock response of the relative displacement of the head actuator arm assembly is studied.