Abstract: An advanced time-domain dynamic model for the investigation of semiconductor optical amplifiers(SOA) is presented.The model accounts for the ultrafast gain dynamics,the gain saturation and the pulse distortion induced by carrier density pulsation(CDP),carrier heating(CH) and spectral hole burning(SHB),the focus is on the ultrashort pulse gain saturation in SOA.It is shown by numerical simulation that there is a critical pulsewidth around 10ps,below which the intraband effects cannot be neglected.