Mg-doped ZnO (ZMO) nanocrystal (NC) has been widely used as an electron transport layer (ETL) in quantum dot (QD) light-emitting diodes (QLEDs). However, its abundant oxygen vacancy (O_V) defects, high chemical activity and poor reproducibility of synthesis severely degrade device performance and shelf stability, limiting the practical application in display. To address these challenges, high-quality Sn⁴⁺/Mg²⁺ co-doped ZnO (ZMSO) NCs were synthesized via a microfluidic (MF) reactor in the present study. This co-doping strategy effectively suppresses the O_V defects formation, increases the conduction band and electrical resistivity to enhance charge balance, as well as enhances the chemical stability of NCs to mitigate unstable chemical reaction during storage. Furthermore, the MF synthesis of NCs ensured consistent reproducibility and scalability. Based on the new ZMSO-based ETL, a remarkable enhancement of ZnSeTe blue QLED performance has been achieved, with a 2-fold increase in external quantum efficiency (EQE) from 9.6% to 19.7% and a 4.6-fold improvement in lifetime (@100 cd·m⁻²) from 3931 to 18,236 h. Notably, the ZMSO-based device exhibited lower efficiency roll-off at high luminance and superior shelf stability compared to ZMO-based devices, offering an effective approach to achieving highly efficient and stable Cd-free blue QLEDs toward practical applications.