This water drainage devices is designed with a theory that is similar to the bend dam, yet not to be pressed but only deliver pressure by itself to allow water to quickly drain off. This design solves the problem of discharged rain water with the retaining wall for many years. The advantage details are as follow: 1. The subject is designed as a hemisphere; 2. The area of water collection adopts ditch type; 3. The plastic gravel is filled in the area of hemisphere of the subject; 4. The arc type helps to lead to he water area; 5. The drain the with elasticity helps to subject in water drainage hole firmly.