Research and advances on technical problemsrelated to composite insulators greatly promote the constructionand development ofultra high voltage (UH)transmissions in China. The fabrication technology, material, testingmethods, ageing, mechanical performances, shed structure,breakage due to embrittlement and ageing of compositelators are key items in the research field of high voltageinsulation all the time. The shed crack under stronew problem and there is no report of relative research and nopointed clause in IEC standard and Chinese National Standardas well. Utilizing wind tunnel experiment and simulation theimpact of the fluid on composite insulator understrong breezeis revealed and the failure mechanism of composite insulatorshed is expounded. It is discovered by the research that whenthe wind speed is higher than 35 m/s, high-frequency vibrationwith large-amplitude occurs on the sheds of the compositeIsulators used in the experiments, and it makes serious stressconcentration at the roots of the sheds. and the roots ofinsulator sheds are in the state of stress fatigue during thecyclic load, thus the micro-cracks appear at the roots of thesheds and then expand into deep cracks, which penetrate shedroots, and finally lead to shed cracks