Recently, zero-dimensional (0D) lead-free Cs3Cu2I5 materials have become one of the most attractive research hotspots in the optoelectronic field owing to their simple preparation method and long-term air stability. However, the photoluminescence quantum yield (PLQY) of the blue luminescence of Cs3Cu2I5 still lags behind those of the green and red luminescence of lead halide perovskites. Herein, 0D lead-free Cs3Cu2I5 films modulated by hydroiodic acid (HI) with a high PLQY of blue luminescence are addressed. The results show that the particle size of the Cs3Cu2I5 films decreases as the added amount of HI increases, from 223.3 (without HI) to 200.7 nm (with 0.076 mmol HI). The Cs3Cu2I5 films with 0.038 mmol HI display the lowest root-mean-square roughness of 14.7 nm. Moreover, the as-prepared films all exhibit blue luminescence at 445 nm, and the PLQY of Cs3Cu2I5 films reaches 85.3% when 0.038 mmol HI is added. Most importantly, the obtained films demonstrate long-term air stability under an ambient atmosphere, and the crystal structure and PLQY of Cs3Cu2I5 films have no obvious variation after 150 days. This air-stable lead-free blue luminescence with high PLQY provides a new strategy to promote the blue luminescence optical property of halide perovskite materials. [ABSTRACT FROM AUTHOR]