The experiment result of gain performance of JG2 Nd:glass and the output of whole furniture was introduced which would be applied in 400 mm aperture slab amplifiers from high power inertial confinement fusion laser equipment. A 4×2 combined slab amplifier system of three-pieces-length and 400 mm-aperture was used to study the gain function. The gain and output experiment results of JG2 Nd: glass show that the small signal gain coefficient reaches 5.37%/cm, and the gain multiple of small signal is 1.284 times/pass when the system operating voltage is 31 kV. The thermal recovery time is about 2 h after cooling with clean and dry gas at a velocity of 0.3 m/s. Besides, experimental results acquired by Integration-Test-Bed (ITB) laser facility show that the optimal combination of JG2 and N41 Nd: glass reaches the maximum output energy of 21.3 kJ/1053 nm, and has been operated stably for more than 500 shots, without any faults such as abnormal cladding layer and material body damage.