Accumulation effect of multi-shot laser-induced damage to optical coatings was investigated.Laser damage testing was performed at 1064 nm with nanosecond pulses at a pulse length of 12 ns,using an accurate and reliable measurement system. Damage threshold and the number of shots were studied by a statistical method. Comparing the case of single-shot with multi-shot laser damage,it is known that microdefects in the optical coatings were responsible for multiple pulses damage to mirrors. The damage mechanism was used to explain the damage process. The expression for the damage threshold and the number of shots was founded. Nomarski microscope was employed to observe laser-induced damage morphology after laser irradiation.