Large-diameter K(H_xD_1-x)₂PO₄ (DKDP) crystals have been widely applied for the optical parametric chirped pulse amplification and broadband UV generation in the superintense ultrafast lasers and the inertial confinement fusion facility. Herein, we systematically investigated the noncritical phase-matching wavelength properties of KDP-family crystals at room temperature and demonstrated a precise characterization method for the deuterium contents of large-diameter DKDP crystals based on the ~245 and ~215 nm deep-UV sum-frequency generation (SFG). This SFG measurement method overcomes the low accuracy disadvantage of the Raman spectroscopy, especially for the DKDP crystals with deuterium contents of >90%. Full-range (0~100%) measurement with a precision of ~0.148 %/cm^-1 was achieved, which is an enhancement of more than one order of magnitude over conventional spectroscopic schemes. This work not only has the potential to be a standard method for the deuterium content characterization of DKDP crystals, but also paves the way for the energy improvement and applications of the tunable deep-UV radiation by fully utilizing the large-diameter characteristics of KDP-family crystal.