Lead-free metal halide double perovskites have emerged as promising scintillators owing to their superior optoelectronic properties, low-cost and solution processability. However, it is still challenging to develop high-performance flexible X-ray scintillators based on the B(I)-site alkali metal modulation in lead-free double perovskite nanocrystals (NCs). Herein, a series of novel Cs₂B(I)GdCl₆ (B: Li, Na, K) of double perovskite A₂B(I)M(III)X₆ NCs structures are successfully synthesized, the optical and scintillator properties of which are significantly sensitive to the B(I)-site alkali metals with the doping of Sb³⁺_. They showed efficient self-trapped exciton (STE) emission with high photoluminescence quantum yield (PLQY) and exhibited the highest X-ray detection limit (86 nGy_air s⁻¹) and excellent spatial resolutions (>15 lp mm⁻¹). The femtosecond transient absorption measurement and theoretical analysis further revealed that the B(I)-site alkali metal fundamentally balanced the exciton–phonon coupling with appropriate STEs formation energy barrier and the electron localization and thus improved the optical and scintillator properties. This B(I)-site alkali metal engineering offers a strategy to develop bright luminescent double perovskite NCs for X-ray detection and imaging devices.