A collapsible umbrella with a self-opening mechanism is provided. The collapsible umbrella includes a stretchable multi-segment shaft having a top end and a bottom end; an upper fixed hub mounted axially on the top end of the shaft, the upper fixed hub being formed with a plurality of alignment slots and a threaded hole, the threaded hole allowing a finishing cap of either an inner-threaded type or an outer-threaded type to be mounted thereon; a running hub slidably mounted on the shaft; a collapsible frame of a plurality of ribs including a set of upper ribs, a set of inner branch ribs, a set of middle branch ribs, a set of linkage ribs, and a set of outer branch ribs, wherein each of the middle branch ribs is formed with a substantially U-shaped cross section to form a slot and integrally formed with a retaining piece; a fabric held on the collapsible frame; and elastic means which allows the collapsed umbrella to be thereafter expanded in a self-opening manner. The foregoing collapsible umbrella allows a more secured and reliable self-opening mechanism for the umbrella. Moreover, it allows either an inner-threaded type or an outer-threaded type of finishing cap to be mounted on the upper fixed hub of the umbrella for versatile utilization.