Ultrasmall CsPbI3 perovskite quantum dots (QDs) are the most promising candidates for realizing efficient and stable pure-red perovskite light-emitting diodes (PeLEDs)1, 2, 3, 4–5. However, it is challenging for ultrasmall CsPbI3 QDs to retain their solution-phase properties when they assemble into conductive films, greatly hindering their device application3,6. Here we report an approach for in situ deposit stabilized ultrasmall CsPbI3 QD conductive solids, by constructing CsPbI3 QD/quasi-two-dimensional (quasi-2D) perovskite heteroepitaxy. The well-aligned periodic array of edge-oriented ligands at heterointerface triggers a substantial octahedral tilting in a critical layer thickness of CsPbI3 QDs, which heightens the Gibbs free energy difference between the tilted-CsPbI3 and δ-CsPbI3 leading to thermodynamic stabilization of CsPbI3 QDs. The approach allows us to fabricate stabilized CsPbI3 QD conductive films with tunable emission covering the entire red spectral region from 600 nm to 710 nm. Here we report the pure-red PeLEDs with narrow electroluminescence peak centred at 630 nm, matching the Rec. 2100 standard for ultrahigh-definition display. The champion device exhibits a certified external quantum efficiency of 24.6% and a half-lifetime of 6,330 min, ranking as one of the most efficient and stable pure-red PeLED reported to date. The approach is also compatible with large-area manufacturing, enabling 1 cm2 PeLED to exhibit the best external quantum efficiency of 20.5% at 630 nm.