The impact of laser conditioning (LC) fluence and pulse duration on nanosecond (ns) laser damage performance of deuterated potassium dihydrogen phosphate (DKDP) crystal is studied. The result shows that higher LC fluence leads to a better damage resistance. In general, the sub-nanosecond LC effect is better than the nanosecond LC. However, in the range of 0.3 ns to 0.8 ns, the pulse duration has no obvious impact on the LC effect. An ultra-fast process characterization technology is employed to demonstrate that the cleaning effect of the protuberance defects on the surface is one of sub-ns LC mechanism. Eventually, a couple of optimized LC parameters that doubled the maximum damage threshold of DKDP crystal is proposed.