In the experiment of HfO2/SiO2 coatings damaged by femtosecond single pulsed laser, the evolution of coating ablation along with increase of the laser energy was observed. The research shows that coating defects even in nanoscale can induce local light field intensification, which superposes on the interference field. As a result, the coating damage in threshold phase is mainly controlled by the particle defects even when irradiated by femtosecond laser. Because of the intrinsic damage characteristic of femtosecond laser to the coating materials including the defects of even distribution, the damage thresholds of these coatings are relatively deterministic. With the increase of pulse energy, the outline of damaged area becomes clearer. In this condition, the contribution of interference field is increasingly dominant compared with the local light field caused by defects, and the phenomena of coating spalling cover up the impact of defects.