A scanning spectral filter method is proposed for improving the intensity contrast originating from the phase ripple in the femtosecond chains, which is made using the character that the instantaneous frequency varies with the time approximately linearly for the chirped pulse in the time-frequency domain. The scanning spectral filtering for reducing the pre-pulse and post-pulse intensity is analyzed in the time-frequency domain by using the short-time Fourier transformation method. Results show that the coherent intensity pulse contrast can be improved by 0.41F2 for the chirped pulse or the compressed pulse, and the transmission efficiency of the chirped pulse can exceed 90% when the synchronizing time of the jitter ranges from -2 ps to 2 ps and the chirp rate p from 0.9C/T2 to 1.1C/T2.