Utilizing the scientific charge-coupled device (CCD) and Hartmann test system, the gain distribution uniformity of large aperture slab amplifiers is measured on the integration test bed and processing methods are put forward for these two methods. The effects of various measuring methods and post-processing methods on gain uniformity are compared in detail by numerical calculation and analysis. The results show that the measuring randomness can be avoided and spatial noise of gain distribution can be minimized when measuring the gain uniformity by the scientific CCD with multi-shot averaging smoothed method. The results also show that less lattice number reduces the gain roll-off at the edges of the aperture when measuring the gain uniformity by Hartmann, which indicates a lattice number of over 50×50 is proposed.