A series of La₂Mg_4/3Sb_2/3-xO₆∶xMn⁴⁺ red phosphors were prepared via traditional high-temperature solid-reaction method, and their structure, morphology and spectral properties were investigated. The results show that La₂Mg_4/3Sb_2/3O₆ belongs to the double-perovskite structure as La₂Mg_4/3Nb_2/3O₆ does, and the incorporation of Mn⁴⁺ has no influence on the structure of the phosphors. La₂Mg_4/3Sb_2/3-xO₆∶xMn⁴⁺ phosphor exhibits two broad excitation bands at 360 and 510 nm, corresponding to the ⁴A_2g→²T_2g and ⁴A_2g→⁴T_2g transitions of Mn⁴⁺ respectively. Upon 360 nm excitation, La₂Mg_4/3Sb_2/3-xO₆∶xMn⁴⁺ phosphor gives rise to a intense red emission band at 710 nm, which belongs to the ²E_g→⁴A_2g transition of Mn⁴⁺. The optimal concentration of Mn⁴⁺ in La₂Mg_4/3Sb_2/3O₆ host was determined to be 0.004, with quantum yield and color purity up to 89.6% and 99%, respectively. A crystal field analysis for Mn⁴⁺ in the La₂Mg_4/3Sb_2/3O₆ matrix was conducted based on the excitation and emission spectra, and the crystal field parameters D_q, B and C were calculated. The temperature quenching process of La₂Mg_4/3Sb_2/3-xO₆∶xMn⁴⁺ phosphor was analyzed with the configurational coordinate diagram of Mn⁴⁺, and the activation energy was calculated to be 0.355 eV. Finally, a comparison between the emission spectra of La₂Mg_4/3Sb_2/3-xO₆∶xMn⁴⁺ phosphor and the absorption spectra of phytochrome P_R and P_FR was provided, and the application prospect of La₂Mg_4/3Sb_2/3-xO₆∶xMn⁴⁺ phosphor in LED plant illumination was preliminarily evaluated.