To analyze the mechanism of damage threshold enhancement after laser conditioning, the fluorescent and stimulated Raman scattering properties of unconditioned and laser-conditioned KD₂PO₄ crystals are compared in detail. It is revealed that the intensity of fluorescence decreases significantly, especially fluorescence <400  nm, after nanosecond laser conditioning, and the change of the stimulated Raman scattering peak at 921  cm^−  1 is very weak. Moreover, the intensity of fluorescence further decreases after subnanosecond laser conditioning. A sharp decrease in the fluorescence intensity <400  nm reflects a change in energy levels of the electron defects in a crystal. The Raman scattering proves very weak change of the PO₄ vibrational modes. Furthermore, a laser-conditioning mechanism is discussed in combination with electronic transition and thermal processes.