Pulse-width of ultrashort laser is a very important parameter for the wide applications of ultrashort pulse. Various methods have been proposed to measure pulse-width, such as second harmonic generation method, third harmonic generation method and frequency resolved optical gating method. Despite of the benefits they hold, they share a vital drawback: they all employ nonlinear effects of ultrashort pulse, which means that the input power must be large enough to generate nonlinear effect, and the central wavelength of the pulse must correspond to the sensitivity range of nonlinear crystal. In this paper we present a new method of measuring pulse-width based on the Talbot effect of ultrashort laser pulse. The ultrashort pulse can be treated as a series of waves of different wavelengths, so at certain Talbot distances, the diffraction of the ultrashort laser pulse reshapes the energy distribution. Thus it gives us the possibility to determine the pulse-width by means of the diffraction. Taking advantages of Talbot effect, the method has the features of large measuring wavelength range, accurate measurement and simple structure. As the basis of the method is diffraction rather than nonlinear effect, all the problems related to nonlinear effect are avoided, such as high incident power and wavelength requirement. Both single and multiple shot pulse are suitable for this method. Numerical analysis has shown that pulse-width from 1 to 100fs can be measured with error of less than 1fs, at wavelength of 800nm. As there is just one Talbot grating between the light source and the detector, we can make the conclusion that this very simple method of pulse-width measuring should be highly interesting for practical application.