In order to obtain a shorter pulse in a chirped pulse amplification system, the spectrum of the seed pulse is shaped to compensate the effect of gain narrowing, to make sure the amplified pulse has enough spectral width and given distribution. Supposing the spectral distribution of the chirped pulse after amplification is be Gauss or super-Gauss distribution, the inverse calculation on 100 J Nd∶glass chirped pulse amplication system is carried out. It is found that the energy, bandwidth of the seed pulse and the signal-to-noise ratio of the compressed pulse are strongly related to the distribution of the amplified pulse spectrum. After spectral shaping, the amplified pulse spectrum is second-order super-Gauss distribution, and 400 fs pulses are obtained with high signal-to-noise ratio, 33 μJ seed pulse energy and 14 nm bandwidth. The results provide a theoretical base for the spectral shaping experiment of 100 J system.