In order to obtain a higher power laser pulse in the terminal, it is necessary to conduct spectrum shaping before the seed pulse being injected into the main amplifier chain to compensate the effect of gain narrowing. Based on the spatial distribution of the chirped pulse spectrum in the stretcher, a scheme to shape the spectrum of a narrow-band laser pulse by using a quartz crystal plano-convex lens spatial light modulator is proposed. The flat-topped spectrum is obtained at the output of the stretcher. The spectrum profile can be easily switched from one profile to another by regulating the polarizing direction of the polarizer.