Eu3+-doped red-emitting ceramics of Eu3+-doped La3Mg2NbO9 were prepared via typical solid state. X-ray diffraction and scanning electron microscope were utilized to characterize the ceramics. The photoluminescence excitation and emission spectra, the fluorescence decay curves, and color coordinates were investigated. The concentration quenching of the samples were discussed as well. The microstructures of the ceramics were discussed according to the spectral properties of probe ions of Eu3+, for example, substitution sites for Eu3+, inhomogeneous broadening and splitting of the emission bands, nonexponential decay, 5D0→7F0 emission transition, distorted symmetry sites, etc. The crystal structure of La3Mg2NbO9 is heavily distorted due to the mixed occupation of Mg and Nb on B sites. Eu3+ ions only substitute La3+ sites and Eu3+ ions (or rare-earth ions) are arranged in the heavily disordered environments over the whole structure in La3Mg2NbO9.