A simple model is presented to analyze the spectral shape and bandwidth dependence of the pulsecontrast and compressed pulse width in the chirped pulse amplification. The parameters of the 30 fslaser system are demonstrated as examples. Comparing with Top hat, Lorentzian, Sech2, Gaussian, 2ndSuper Gaussian and 10th Super Gaussian spectral pulse shape, the 2nd Super Gaussian spectral pulseshape can obtain better contrast in the case of less spectral bottom width. Those results are helpful tofind an optimized spectral shape in the chirped pulse amplification.