Yb³⁺ ions co-doping was attempted for the first time for sensitizing Er³⁺ ions and increasing the 3.1 µm fluorescence, originating from the transition from Er³⁺: ⁴S_3/2 → ⁴F_9/2. Under a 980 nm LD excitation, strong 3.1 µm fluorescence in the mid-infrared (MIR) region and the related energy level change process were investigated. An enhanced 3.1 µm emission was attained from the Er³⁺/Yb³⁺ co-doped tellurite glass with the increment in Yb₂O₃ content, which demonstrated that the Yb³⁺ ions may act as the sensitizer to the Er³⁺ ions, thereby providing an effective energy transport channel from the ²F_5/2 state of Yb³⁺ ions to the ⁴S_3/2 state of Er³⁺ions. Moreover, the measured emission band full width at half maximum reaches about 164 nm along with the maximum emission cross-section (2.88 × 10⁻²¹ cm²). The outcome of our work suggests that upon co-doping with Er³⁺/Yb³⁺, the tellurite glass may function as a prospective material for fabricating the matrix of 3.1 µm MIR lasers.