Potassium dihydrogen phosphate (KH₂PO₄: KDP) crystals are extremely important nonlinear optical crystals with high laser damage thresholds, and are used as frequency doubling and switching devices in inertial confinement fusion. In the present work, the effect of transition metal ion doping on the structural, electronic, and optical properties of KDP crystals are systematically investigated by using first-principles calculations based on density generalized theory. Our results show that the pure KDP has a band gap of 5.65 eV, which is significantly reduced to 1.37 eV for the doped Mn. Fe-doped KDP crystals show half-metallic properties with 100% spin polarization, while Mn-doped KDP show semiconductor properties. For the case of two transition metal doping, the formation energy of Fe ion defects is smaller than that of Mn defects. Therefore, Fe ion defects are more likely to form than in Mn-doped KDP. The maximum energy loss of pure KDP is 25.82 eV, while, when doped with Mn, the energy loss shifts to 25.56 eV, and, when doped with Fe, the energy loss is reduced to 25.14 eV. With the increase of the radius of the doped ions, the peak experiences an obvious red shift. The same results are expected to provide theoretical guidance for the fabrication of KDP crystals.