We study the influence of alkali oxides on the near-infrared (NIR)-emitting thermal stability of Bi-doped R2O–SiO2–B2O3–Al2O3 (R = Li, Na, K) glasses below Tg. Results show that undergoing heat treatment, re-markable luminescence quenching occurs for the glasses containing Na2O and K2O due to the formation of Bi metallic colloids, whereas the glass with Li2O shows much better thermal stability. These changes can be understood by the tendency of modifier cations with lower mobility and higher tightness network to restrain the transport of Bi-related NIR-emitting centers. The results provide a scientific reference for composition design of Bi-doped optical fiber.