Experimental research about the transmission wave-front characteristic of high power slab gain module in resonator is carried out. The measurement of transmission wave-front of slab gain module under different power extraction status is finished through changing the parameters of the resonator. The experiments indicate that the power extraction status influences the transmission wave-front of slab evidently. Compared with the plano-concave resonator and the asymmetric output plano-plano resonator, the transmission wave-front distortion of the slab gain module is the minimal with the symmetrical output plano-plano resonator, the peak-to-valley (PV) value of the wave-front distortion is less than 1.5 μm when the pumping power is 10.8 kW.