The maximal output fluence of high-power laser systems remains limited by KH2PO4 (KDP)-based optical components used to convert the infrared pulses to 3ω pulses. The correlation between optical defects (scales of a few micrometers) on KDP crystal surfaces and damage initiation was systematically investigated by combining laser-induced fluorescence (FL) and light-scattering microscopies incorporated an in-situ damage testing facility (wavelength at 355 nm and pulse width of 7 ns). We demonstrated that the surface defects featuring both FL and light-scattering have a high propensity to damage. The scattering feature of the FL precursors was found to be a key factor for damage initiation, whose physical mechanism was thoroughly revealed. The results further implied that the removal of such damage precursors can greatly improve the damage resistance of KDP-based crystals. Considering that this combined imaging technique is noninvasive, the results establish this methodology as an attractive and non-destructive tool for evaluating and improving the surface damage performance of KDP-based crystals used in high-power lasers.