Abstract: The modulation characteristics of the liquid space light modulator was discussed, and its application in the large mode area photonic crystal fiber ultrafast laser amplification system was studied. The results indicated the real time control on pulse shape was realized. The influence of pump power on the spectral and temporal shape of strongly linearly chirped square pulse after amplification was discussed. By using a spectral pulse pre-shaping of the seed pulse before the power amplifier, the square pulse with 15-ps pulse duration, 13-nm bandwidth at 1 031-nm central wavelength was obtained. The results also demonstrate the significant role of liquid space light modulator in compensating pulse distortions induced by the asymmetric gain, by a pre-optimization of the seed pulse spectrum before amplification.