One of the main problems occur during inertial confinement fusion (ICF) laser facility’s long-term operation is the gain degeneration of the 400 mm aperture slab amplifier,which will affect the output of the facility and the laser beam quality. A study on gain degeneration causing by several factors was carried out and a normalization theory model from all the factors has been built. The test was accomplished on two groups of 400 mm aperture, 4×2 composition multi-segment slab amplifier with each group includes 9 slabs. The gain degenerating rate was about 10.2% after 10 years, 3 000 shots of work which is in accordance with the theoretical predication. A maintance project for the large aperture slab amplifier has been drawn up to keep the gain degeneration less than 1.5% during long-term operation of the ICF facility.