Potassium dihydrogen phosphate (KDP, KH₂PO₄) is a representative hydrogen-bonded inorganic crystal widely used in high-power laser frequency conversion and electro-optic modulation, yet its deep-ultraviolet optical response and service reliability still require further improvement. In this work, first-principles calculations based on density functional theory were performed to systematically investigate the synergistic regulation of the structural stability, electronic structure, optical response, and mechanical properties of Group-I alkali-metal-doped KDP crystals, K_1-xM_xH₂PO₄ (M = Li, Na, Rb, and Cs; x = 0.25, 0.50, and 0.75). Geometry optimization, ab initio molecular dynamics, and elastic-eigenvalue analysis indicate that all doped configurations remain thermodynamically, dynamically, and mechanically stable and preserve the KDP-type framework within the investigated composition range. The calculated electronic structures show that all systems retain wide band gaps above 5 eV, while both the gap magnitude and gap character can be tuned by the dopant species and concentration. Li- and Na-doped systems remain indirect-band-gap materials, Rb doping produces an evident quasi-direct-gap tendency, and Cs doping at x = 0.25 and 0.75 induces a distinct indirect-to-direct band-gap transition. Density-of-states analysis reveals that the valence-band maximum is mainly dominated by O-2p and P-3p states, whereas the conduction-band minimum is primarily composed of P-3s and P-3p states with minor O-2p contributions. The alkali-metal dopants do not directly dominate the band edges; instead, their main role is to regulate the local crystal field and lattice distortion and then redistribute the O-P framework states near the band edges, especially for the heavier Rb and Cs dopants. Optical calculations further demonstrate that Rb- and Cs-doped systems exhibit stronger modulation of the deep-ultraviolet absorption edge, extinction coefficient, refractive index, and dielectric polarization than Li- and Na-doped systems. Mechanical analysis based on the Hill elastic moduli and Pugh ratio shows that the doped crystals generally maintain moderate stiffness with adjustable ductility; among them, K_0.5Na_0.5H₂PO₄ shows a relatively good balance between rigidity and ductility, whereas Rb- and Cs-doped systems provide broader composition windows for enhancing optical performance without obvious mechanical deterioration. These results indicate that Group-I alkali-metal doping offers an effective route for the cooperative optimization of band-gap characteristics, deep-ultraviolet optical properties, and mechanical reliability in KDP-based crystals and provides theoretical guidance for the compositional design of high-power deep-ultraviolet nonlinear optical materials.