Based on Kogelnik′s coupled-wave theory, we derive the coupled-wave equations of a volume holographic grating (VHG) illuminated by ultra-short pulsed laser, and obtain its diffraction electric field and instantaneous diffraction intensity. It is found instantaneous diffraction intensity of volume grating is affected by the VHG′s structural parameters, such as the grating spacing, grating thickness, and the refractive-index modulation of the VHG. By changing these parameters, diffracted characteristics could be controlled and this may be used in the pulse shaping. In simulation, we also find group time delay of the diffracted beam with respect to the readout pulse in the negative time axis and this time delay are affected by the grating spacing and thickness. With the concept of group velocity, we give a quantitative analysis about this time delay.