The Fresnel diffraction equation of a grating under chirped ultrashort pulsed laser illumination is given through Fourier spectrum analysis, and the Talbot effect of the grating is studied using the equation. Numerical calculation shows that the intensity distribution of the grating at Talbot distances is related to both the width and the chirp parameter of the ultrashort pulse. According to the intensity change of the grating, a simple method is put forward to check whether an ultrashort pulsed laser contains chirp information.