A theoretical model of chirpedpulseamplification is presented, and based on which we give the numerical analys is on the evolution of stretched pulse in the amplifier. Amplitude and phase modulation including: gainnarrowing, selfphasemodulation, gainsaturation and higherorderdispersions experienced by the pulse in the CPA system are resp onsible for the distortions on the temporal shape, power spectra and recompressed autocorrelation trace of the amplified pulse. In addition, for the first time the contribution of stretching chirps from the stretcher on the distortions of the pulse is studied in detail.