An equation for calculating the Fresnel diffraction field of a grating under short-pulsed light illumination is derived. As an example, the equation is used to analyse the diffraction field of a rectangular grating with the opening ratio 1/4. The following results are found: Talbot self-imaging effect will appear when the full width at half-maximum of the incident short-pulsed light (FWHM) Δτ is within picoseconds and it will be gradually diminished at Δτ becomes small. When Δτ is within a few decade femtoseconds, the maximal intensity of the diffraction field at the first Talbot distance decreases along with the decrease of Δτ.