Potassium dihydrogen phosphate (KDP) was grown by the slow cooling method together with seed rotation, and the effect of different concentrations of L-phenylalanine (L-Phe) doping on the optical and thermal stabilities of KDP was also investigated. The results of the study showed that: (1) L-phenylalanine doping had no significant effect on the microstructure of the KDP crystals, but an appropriate amount of doping could accelerate the growth of the (100) crystal surface, modifying the growth manner and morphology of the KDP crystals. (2) The optimal amount of L-phenylalanine doping (1 mol%) increased the light transmission intensity of KDP but had no significant effect on the thermal stability and laser damage threshold. (3) L-Phenylalanine can only be incorporated into KDP crystals in the form of interstitial doping or inclusions, affecting the thermal and optical properties of crystals through the regulation of the growth manner.