Pulse evolution after the acousto-optic deflector (AOD) and its dependence on AOD parameters is not clear, but is important to AOD applications in scanning microscopy. By considering the abnormal Bragg AOD as a combination of a dispersion media and a diffraction grating with Littrow structure, the dispersion characteristics of AOD when scanning Gaussian laser beam of femtosecond pulses were analyzed. Pulse width about AOD parameters was explored theoretically and verified with experiments. It is found that after exiting AOD, the pulse width of laser beam either merely decreases or first decreases and then increases before it finally goes steady with the propagation distance, depending on the interaction between the material dispersion and angular dispersion.