Our product of water drainage device is designed by the theorem of similar to bend dam, not be pressed but only deliver pressure by itself, to enhance drain off water fast. Solve the problem of discharge rainy water of retaining wall many years. The advantage is detailed as follows: 1. The subject is designed as type of hemisphere 2. The area of collection water adopt ditch type 3. The plastic gravel is filled in the area of hemisphere of subject 4. The arc type of leading water area 5. The drain pipe with elasticity to press subject in water drainage hole firmly