The frequency domain distribution of amplified pulse modulated by the acousto-optic programmable dispersive filter is measured on the 200 TW laser system in Peking University. A spectral modulation function related to the pulse central wavelength and spectral bandwidth is established to fit the measured spectral data. The function is simple and suitable for different laser systems. Based on the numerical simulation of the amplifier output pulse′s spectra after the seed pulse modulation, the time-domain characteristics of the ordinary seed pulse and the spectrum modulated pulse through the amplifier are compared. The effects of spectral modulation on the peak power of the final output pulse are discussed. The result shows that the spectral modulation can lead to the generation of sidelobes and reduce the effective energy and femtosecond contrast of the system output pulse. When the output pulse has a flat-top spectrum, its effective peak power is the largest.