The invention relates to a method for online detecting optical element damage of a multi-pass amplification laser system, and belongs to the technical field of laser detection. The method comprises the following steps: carrying out bright field illumination through pre-amplifier repetitive-frequency laser which is injected in the multi-pass amplification laser system, switching states of a space filter, collecting near-field images at an output end of the laser system, observing the near-field images under different states of the space filter, judging whether an optical element is in damage or not, and determining the positions and the size of damage spots according to a variation situation of the near-field images and offsets and patterns of different multi-pass amplification damage spots. The method for online detecting the optical element damage of the multi-pass amplification laser system, provided by the invention, has the characteristics that the equipment cost is low, the structure is simple, the operation is convenient, the online detecting time is effectively reduced, and the flexibility is high, and is suitable for online detection on the optical element damage of the high-power multi-pass amplification laser system.