A monitoring and compensation system which is built for an amplifier that can generate flat-top laser beam is described. The amplifier uses a multi-level amplification structure. It can output 5 J energy of a single pulse, and the spot size of the flat-top laser beam is 50 mm×50 mm. The liquid-crystal valve in the amplifier can modulate energy distribution of the output spot effectively. The monitoring system is combined with the spatial filter in the amplifier, and by the theory of Fourier-image transfer, it can image energy distribution of the liquid-crystal valve on the monitoring CCD. The compensation system effectively improves the quality of the output laser beam by feeding back the output spot of the amplifier on the liquid-crystal valve. Experimental result shows that energy distribution of the shaped beam is more uniform, and the near-field modulation of the flat-top laser beam generated by the amplifier is 1.64, the softening factor is 0.053. The beam-quality control and improvement are well realized by the monitoring and compensation structure.