The transmission functions and the characteristics of transmission spectra of phase-shifted fiber gratings are analyzed based on the transmission matrix method and the coupled mode theory. The results show that the phase-shifted fiber gratings have superior integral property, and the integral order is proportional to the number of inserted phase-shifted points. The pulse shaping structure is designed based on the first-order integral characteristic of phase-shifted fiber grating, which can shape the Gaussian ultrashort optical pulse into a flat-topped pulse or a symmetrical triangular light pulse. The system can output asymmetric triangular light pulses by further adjusting the weighting coefficients in the proposed structure. When the pulse width of the input Gaussian light pulse fluctuates ±10%, the flat-topped pulse and triangular light pulse with high quality can be obtained, which proves that the designed pulse shaping system has high stability.