Using Liquid Crystal Spatial Light Modulator (LC-SLM) as a beam shaping device to improve beam quality in high-gain amplification system is reported. 1.6 nJ injected small-size signal Gaussian beam can be amplified to 5 J by 4 stages amplification, and finally output beam is a 50mm×50mm square spot with flat-top intensity distribution. In the amplification system we designed, LC-SLM is placed after the second level of amplifier, where the signal laser energy is about 20mJ, and beam size is 10mm×10mm. The structure of Fourier image transfer is also implemented in this amplifications system to be capable of maintaining high-quality image transmission in the amplification process. The LC-SLM as an object, is imaged by beam expand lenses and spatial filters lenses in the amplifications system to get good quality of imaging. By catching output spot and making a feed-back, transmission efficiency of each pixel on LC-SLM is modulated, high energy density area can be decreased to realize flat-top intensity distribution. A spot modulation function is defined as, using the maximum grey value on spot area divided by the average grey value of the image after background correction. By this, amplified laser obtains the spot modulation of 1.24 on central 90% area of the spot. Furthermore, un-uniform distribution on the full spot, soften effects of spot edge, and output beam shape can also be optimized by the LC-SLM shaping scheme in the amplification system.