A Yb∶YAG thin disk laser with a novel four-pass pumped structure and direct water cooling device was designed. The multi-mode pump radiation was from a fiber-coupled 940 nm diode laser. The Yb∶YAG disk with 10 mm diameter and 500 μm thickness was employed. The Yb3+ atom fraction was 10%. The direct cooling arrangement was put forward,tap water was contacted with the Yb∶YAG through a V groove. The four-pass pump coupling system was well designed with a pair of rectangular prism and a focusing lens which had a diameter of 50 mm and focus of 50 mm. Then simulated the stability of the resonator and calculated the radius of the laser spot in different position of the resonator with the different cavity length. Different types of laser cavity were set up. With the output coupling rate of 5%,the maximum output power of 3.28 W at 1 031 nm was obtained in F-P resonator. The beam quality M2x=1.79, M2y=1.86. The slope efficiency was 20.5%.