A series of Pr$^{3+}$, Gd$^{3+}$ and Pr$^{3+}$–Gd$^{3+}$-doped inorganic borate phosphors LiSr$_4$(BO$_3$)$_3$ were successfully synthesized by a modified solid-state diffusion method. The crystal structures and the phase purities of samples were characterized by powder X-ray diffraction. Surface morphology of the sample was studied by scanning electronic microscopy (SEM). The optimal concentrations of dopant Gd$^{3+}$ ions in compound LiSr$_4$(BO$_3$)$_3$ were determined through the measurements of photoluminescence (PL) spectra of phosphors. Gd$^{3+}$-doped phosphorsLiSr$_4$(BO$_3$)$_3$ show strong band absorption in UV spectral region and narrow-band UVB emission under the excitation of 276 nm was only due to ${}^{6}$P$_J$ $\to$ ${}^{8}$S$_{7/2}$ transition of Gd$^{3+}$ ions. The effect of Pr$^{3+}$ ion on excitation of LiSr$_4$(BO$_3$)$_3$:Gd$^{3+}$ was also studied. The excitation of LiSr$_4$(BO$_3$)$_3$:Gd$^{3+}$, Pr$^{3+}$ gives a broad-band spectra, which show very good overlap with the Hg 253.7 nm line. The photoluminescence spectra of LiSr$_4$(BO$_3$)$_3$ with differentdoping concentrations Pr$^{3+}$ and keeping the concentration of Gd$^{3+}$ constant at 0.03 mol have also been studied. The emission intensity of LiSr$_4$(BO$_3$)$_3$:Pr$^{3+}$–Gd$^{3+}$ phosphors increases with increasing Pr$^{3+}$ doping concentration and reaches a maximum at 0.01 mol. From the photoluminescence study of LiSr$_4$(BO$_3$)$_3$:Gd$^{3+}$, Pr$^{3+}$ we conclude that there was efficient energy transfer from Pr$^{3+} $\to$ Gd$^{3+}$ ions in LiSr$_{4−x−y}$Pr$_x$Gd$_y$(BO$_3$)$_3$ phosphors.