The effects of gain saturation (GS) in chirped pulse amplification are studied in this paper. It is found that the compressed signal to noise ratio (SNR) may decrease due to GS and the dispersion added by stretcher. The decreasing corresponds to distortion in frequency domain. The analysis on the stretcher/compressor shows the internal relation between spectroscopy and time intensity. It is advised that frequency mismatch may be used to modify the spectral shape and increase SNR.