The characteristics of Q-switched Nd:YAG laser induced damage to fused optical fibers has been investigated experimentally. A scheme of laser induced damage to optical fiber in a vacuum chamber was presented and carried out.With this scheme, the air pressures on the input face of optical fiber was reduced to 10—100 Pa, laser-induced breakdown on the optical fiber end face was significantly suppressed, the breakdown threshold increased to 185 times that in the atmosphere. The damage morphology of optical fiber end face was analyzed, which was the result of the combined actions of laser pulses standing wave and ablation. The existence of a large number of defects on or under the fiber end face reduces the anti-laser-induced damage ability of the optical fibers. Under the vacuum condition, another damage mode——the fiber “entry” segment damage occurred. This “entry”　segment damage mode occurred at 5—30 mm distances from the input end face of optical fiber, the blasting damage area was 1—3 mm long along the fiber core axis. That the initial reflection of laser beam at the interface of fiber cladding and core causes laser focusing which induced the entry damage, was proved by theory and simulations.