The surface damage morphology of potassium dihydrogen phosphate (KDP) crystal under 351 nm nanosecond laser irradiation is studied, and the formation mechanisms of each damage type are discussed. There are three unique types of KDP surface damage, namely, crack, shell, and crater, under the fluence between 5 and 15 J/cm2. The fracture feature of crack type damage indicates the pure mechanic process during laser exposure. Some cracks result in the upwarp of the material with a height of 0.1-0.5 μm. The shell is a most typical damage morphology with a proportion as large as about 80%. The transverse size of shell ranges from 5 to 82 μm, related to the fluence. The crater has a distinguished core structure related to the high-temperature process. The evidence of dehydration reaction is found in the core by energy dispersive spectrometer semiquantitative analysis. Internal morphology analysis with a focused ion beam suggests that the crack and shell damage originate from the surface or subsurface machining defects, while the crater damage is mainly due to material bulk defects.