Composite insulators promoted development of ultra-high voltage transmission lines and study of the composite insulators is very important at the high voltage engineering. Strong wind area is very special environment at China or the worldwide. Not only the tower strength, conductor galloping and external insulation selection should be considered, but also the insulator structure and the shed materials need to be designed carefully. The sheds cracks of the composite insulators at 750kV AC lines in Xinjiang province is very new topic. It is found that when the wind speed is higher than 30 m/s, the biggest sheds of the tested insulator would vibrate at high frequency and great amplitude. The vibration of the sheds caused the sheds root load stress concentration, which is periodically. Even the stress was lower than the tensile strength of the HTV rubber, the stress fatigue would break the sheds root and the cracks would appear. To restrain the resonance between fluid and the shed, the shed structure parameter and the material used to make insulators sheds should be improved.