A three branch multilayer water cooled Si mirror is designed, which has a spiraiity shape. The characteristics of multilayer composite water-cooled Si mirror are simulated by ANSYS program and studied by experiments. When the flow rate is 1.6 m/s, the water pressure is 1.5×105 Pa, the net absorbed heat is 100 W, the diameter is 80 mm, the largest deform is 0.186 μm The features of cooling process without heating cuased by laser radiation is that the decreasing deform is 0.063 μm in 0.75 s and the deform is 0.038 μm in 10.5 s. The multilayer water-cooled mirror performs well and can be used in high power laser.