In this work, the population bottleneck of Er3+: 4I11/2 -> 4I13/2 was overcome for the first time in heavy Er3+/Pr3+ co-doped TeO2-BaF2-La2O3-LaF3 (TBLL) low hydroxyl fluorotellurite glasses. Infrared emission spectra and fluorescence lifetime decay curves reveal that Pr3+ ions could deplete the electrons from the Er3+: 4I13/2 level faster than those from the Er3+: 4I11/2 under 980 nm excitation. Specifically, the energy transfer (ET) efficiency of the Er3+: 4I13/2 -> Pr3+: 3F3,4 process (ET1) reached 96.27%, while that of the Er3+: 4I11/2 -> Pr3+: 1G4 process (ET2) is only 2.17% in the Er3+/Pr3+ co-doped glass. Additionally, the energy transfer mechanism of Er3+ and Pr3+ ions was investigated using the Dexter theory, where the energy transfer microscopic parameters CD-A are 13.21 x 10-40 cm6/s and 0.89 x 10-40 cm6/s for the ET1 and ET2 processes, respectively. Finally, a numerical simulations laser model was developed to discuss the laser properties of the Er3+/Pr3+ co-doped TBLL fibers. The simulation results indicate that a 2.7 mu m laser with a maximum output power of 2.26 W and slope efficiency of 13.89% could be achieved when the fiber background loss is reduced to 0.5 dB/m. The above results suggest that the Er3+/Pr3+ co-doped TBLL glass has great potential applications in mid-infrared fiber lasers.