Abstract: Influences of various micro/nano processing induced surface defects of KDP crystals on laser damage threshold are analyzed by finite element method, which include residual internal stress, micro-crack and micro-pore. It is found that the defects in machined KDP crystal surfaces may affect the distribution of temperature field and thermal stress field of the machined crystal surfaces, which makes the injected laser beams centralize on a small area around the defects, thus leading to partial melting of KDP crystal and laser damage threshold reduction. The laser damage threshold test experiment of the micro-crack is done, and the experiment result shows that the micro-crack will reduce the laser damage threshold of KDP crystal (reducing about 3 J/cm2) and the result fits the simulation result