The relationship between structures and properties of Kevlar string was studied scarcely before, although the string was used in aerospace, spaceflight, ocean and the like. Kevlar braiding strings were designed and braided with different braiding angle, different core yarn content and different twist, and stretching properties were measured and analysed in the paper. The result shows that the bigger braiding angle is and the more core yarn content is, the higher break load is and the lower break elongation is. Besides, rupture analysis proves that braiding Kevlar bundles take hold of core Kevlar bundles tightly, both form an inseparable integrity almost during stretching. As a result, a special phenomena occurs, core Kevlar bundles are broken into several segments.