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The complex and high‐precision patterning of perovskite quantum dots (PeQDs) is of vital importance for exploring their new functionalities and device applications. In this work, a strategy based on the combination femtosecond (fs) laser‐irradiation and humidity‐control is reported to construct patterns of cyan CsPb(Cl/Br)3 PeQDs in a transparent glass medium. Benefiting from their ionic crystal feature and low formation energy, CsPb(Cl/Br)3 PeQDs can be locally crystallized and decomposed via fs laser irradiation without further heat‐treatment. More notably, it is found that a water molecule is able to affect the growth of CsPb(Cl/Br)3 PeQDs in glass. By modulation of relative humidity in air, the selectively decomposed perovskite structure is spontaneously regenerated, and the highly emissive CsPb(Cl/Br)3 PeQDs can be in situ manufactured in the confined glass region. This allows for reversible luminescence from the same patterns of laser printing‐erasing‐recovering, and the process can be repeated for multiple cycles without destruction of optical performance and robustness. The results provide a flexible method to develop new encryption/decryption technology for information security and anti‐counterfeiting. [ABSTRACT FROM AUTHOR]

• Ultrapure green emissions of glass-stabilized CsPb(Br/I) 3 PeQDs are fabricated. • Quantum yields of CsPb(Br/I) 3 @glass composites are highly dependent on halogen content. • The emissive profiles and FWHMs can be well modified by varying Br-to-I ratio in glass. • The issues of instability, adverse anion-exchange and phase separation of colloidal PeQDs can be solved by glass protection. • Prototypical white LEDs are constructed and applied in backlit display, covering up 92% color gamut of Rec. 2020 standard. CsPbX 3 (X = Cl, Br, I) perovskite quantum dots (PeQDs) have potential application in backlit display. However, they generally suffer from instability and shortage of ultrapure tunable green emissions owing to halogen phase separation. This work reports the realization of elaborately tunable green emissions in glass-protected CsPb(Br/I) 3 PeQDs, which show the advantage of superior long-term stability and enable them to find promising application in ultra-high definition televisions. CsPbX 3 @glass composites are fabricated by a facile in-situ glass crystallization strategy and their emissions within the spectral range of 506–532 nm along with narrow full width at half maximum (FWHM) of ~21 nm can be well modified by simply varying Br-to-I ratio in precursor glass. Benefited from the protection of tight glass network, CsPbX 3 @glass retains 90% luminescence after immersing in water for one month, shows no any loss of luminescence after irradiation by 450 nm LED light and prevents anion-exchange and phase separation completely. As a proof-of-concept experiment, CsPbX 3 @glass-based on-chip white LED devices are constructed and applied in backlit display, covering up 123% and 92% color gamut of NTSC and Rec. 2020 standards in the CIE 1931 color space, respectively. [ABSTRACT FROM AUTHOR]