Y_2−xEr_xMo₄O₁₅ (x = 0.04, 0.08, 0.16, 0.32, 0.64, 2.0) phosphors were synthesised at 700 °C through a solid-state reaction method. The samples were characterised by XRD and emission spectra analysis. Y_2−xEr_xMo₄O₁₅ samples showed a strong green emission (²H_11/2, ⁴S_3/2 → ⁴I_15/2) and a weak red emission (⁴F_9/2 → ⁴I_15/2) under 1550 and 980 nm excitation. The green emission intensity was enhanced increasing the Er³⁺ ion doping content, whereas the red emission was basically unchanged. Thus, Er₂Mo₄O₁₅ exhibited an abnormal green up-conversion luminescence without concentration quenching under 1550 and 980 nm excitation. The dominating green up-conversion luminescence was due to the larger adjacent Er³⁺⋯Er³⁺ distance caused by the special structure of Er₂Mo₄O₁₅, which limited the energy-transfer process and cross-relaxation responsible for the red up-conversion emission. Therefore, the up-conversion luminescence mechanism was excited-state absorption under 1550 nm excitation.