The principle of high-brightness and high-average-power thin-disk laser is introduced and the design principle is put forward. The experiment is carried out with Nd:YAG of 1 mm thickness and 1.4% Nd doping pumped by home-made CW 1 kW 2D-stacks diode array. The pump gain area is 5 mm×7 mm. The peak output power of 397 W is achieved with 39.3% optical to optical efficiency at 20% duty cycle Q-CW running mode.