A series of La3BW1-xMoxO9:Eu3+ (x = 0-0.4) polycrystalline powders were prepared by using solid-state reactions. The phase structure, UV-vis absorption spectra, and photoluminescence properties were studied as a function of the Mo/W ratio. When Mo6+ ions are incorporated into the lattice, the characteristic sharp lines in the excitation spectra of Eu3+ monitored at 617 nm are prominently enhanced, which join the ligand-to-metal charge transfer (LMCT) band of La3BW1-xMoxO9:Eu3+ into a broad band ranging from 250 to 450 nm centered at 375 nm. The intensity of the broad excitation band reaches a maximum when the content of Mo6+ ions increases to x = 0.3. On the other hand, the LMCT band around 306 nm decreases and shifts toward the longer wavelength. These features are advantageous to near-UV or blue light GaN-based LED applications. Orbital population analysis by density functional theory calculation (DFT) reveals that the near-UV excitation of La3BW1-xMoxO9:Eu3+ red phosphor is due to the electronic transition from the O 2p orbital to the W 5d and Mo 4d orbitals, respectively. With the introduction of Mo6+ into the lattice, the band gap of La3BW1-xMoxO9 becomes narrower than that of the pure phase La3BWO9.