A method for fabrication a metal pillar bump packaging structure is provided. The method includes providing a semiconductor substrate; and forming a metal interconnect structure and a dielectric layer exposing a portion of the metal interconnect structure on the semiconductor substrate. The method also includes forming a photoresist layer having an opening with an undercut with a bottom area greater than a top area at the bottom of the opening to expose the metal interconnect structure and a portion of the dielectric layer on the semiconductor substrate; and forming a metal pillar bump structure having a pillar body and an extension part with an enlarged bottom area in the opening and the undercut. Further, the method includes forming a soldering ball on the metal pillar bump structure.