In order to eliminate the risk on the safe operation of power grid caused by the internal defects of composite insulators.A water capsule flexible coupling method based on ultrasonic phased array is proposed for small diameter composite insulator.This method uses the silicone rubber film and the coupling device to achieve good ultrasonic coupling.The defects detection experiments of the plane silicone rubber,composite insulator sheath and core were carried out respectively.Experimental results show that the linear scanning imaging is better than the sector scan and defects of the diameter of 0.8 mm can be detected,which provides a new method for the ultrasonic defects detection of composite insulator umbrella group.For the composite insulator sheath and core,the method of water capsule coupling can detect defects with a diameter of 0.8 mm and 1.0 mm through the phased array sector scanning.It provides the possibility for the on-line detection of composite insulator.