By numerical method, the mechanism of optical pulse compression within the stop band of a fiber grating was demonstrated to be the process of energy storing and tuning out of the stop band. The envolope of energy density, which is a solitary wave like object, is firstly excited by the input pulse, then propagates along the fiber grating and losses energy at the output end to form an output pulse. The temporal width and peak power of the output pulse are determined by speed, spatial width, and peak value of the envolop of the energy density. The self-pulsting effect will induce the formation of multi-pulse output, which can be used for the bigger pulse-compression factor.