Using chirped pulse spectral interferometry, single-shot diagnosis of ultrafast optical transient process can be achieved. The temporal resolution is determined by spectral width while the temporal measuring range is determined by pulse width, and they are both affected by the spectrometer′s work performance. After two linear chirped pulse enter into the spectrometer, the same spectral components will interfere to form spectral interference fringes. If one pulse is perturbed externally, the specific shape of perturbation can be reconstructed according to interference fringe patterns. The process of external perturbation reconstruction is theoretically studied and numerically calculated, which shows that the reconstructed perturbation matches well with true perturbation. With finite spectral recording, reconstructed perturbation signal shows strong oscillation, which is suppressed effectively by smoothing treatment.