Apparatuses and method for real-time measuring ultrashort pulse shape and pulse width. Transient-grating effect on a transparent optical medium is used to generate a reference beam. A black plate with four equal-sized holes divides the incoming laser beam into four beams, one of which is attenuated and introduced an appropriate time delay relative to the other three. The four laser beams pass through a concave mirror and are focused onto a nonlinear transparent optical medium. The three beams without attenuation are used to generate a transient-grating light in the transparent medium. The transient-grating light is collinear and overlapped with the fourth attenuated beam. According to the third-order nonlinear effect, the transient-grating light has a broader spectral bandwidth and more smooth spectrum phase with respect to the incident laser. By measuring the spectral interference, the spectrum and spectral phase may be retrieved by spectral interferometry.