A Yb∶YAG ceramic slab is designed theoretically and fabricated, and the high power laser output is achieved at 1030 nm under room-temperature with InGaAs diode pumping, 1030 nm seed laser injecting and double-pass amplifying. The amplified laser power is 5.97 kW when the total pumping power is 19.98 kW and the injected power of the seed laser is 1.18 kW. The optical-to-optical conversion efficiency is 24.0% and the slope efficiency is 27.9%. The transmission wavefront of the Yb∶YAG ceramic slab is measured and the output powers with different coupling efficiencies and working temperatures are simulated. The experimental results show that the high power laser output can be achieved in Yb∶YAG under room-temperature with high intensity pumping and high intensity seed laser injecting.