The thermal effect of a new large aperture N41 Nd∶glass slab which is applied to inertial confinement fusion (ICF) laser drivers is investigated systematically under the condition of high gain. The experimental results show that the peak-valley value of thermal induced wavefront distortion of two slabs with two pass within the diameter range of 385 mm×385 mm is about 0.84λ (λ=1053 nm) when the average small signal net gain coefficient reaches 0.0525 cm-1, which is in the revise extension of deformable mirror. Following the present cooling schemes, the recovery time of residual wavefront distortion is about 2.5 h, which is satisfied the designed requirement of launching once every 4 h. A minuteness wavefront distortion induced by thermal gas convention exists in the system after launching 7 h. Experimental results are consistent with numerical calculation results. The study provides an experimental reference for the design of the next generation slab amplification system in ICF laser drivers.