Abstract: By theoretical analysis and simulation,laser -induced damage mechanisms of fiber’s end face, entry segment and fiber inside are studied. The admixtures and defects on the fiber end face induce the damage; the first reflections along the fiber core/cladding interface induce the damage of entry segment. The unexpected stress and beam self-focus in optical fibers induce the internal damage. In this case a portion of the fiber cross section experiences significant tensile stresses which aggravate defects within the fused silica, including microcracks at the fiber boundaries which were initially produced during the fiber drawing process. Damage at these locations is always catastrophic, usually resulting in a complete break across the fiber cross section, The methods of increasing fiber damage