Thermally evaporated perovskite light-emitting diodes (PeLEDs) are promising for next-generation displays, yet process-compatible passivation strategies for performance enhancement are still lacking. Herein, an effective in- situ surface passivation strategy using post-evaporated metformin hydrochloride (MFCl) is introduced in co- evaporated PeLEDs. MFCl post-deposited onto the perovskite film induces surface reconstruction, improving surface uniformity and healing grain boundaries. Additionally, MFCl penetrates deeply into the perovskite film, passivating both undercoordinated Pb and halide ions at grain boundaries. By employing this strategy, we achieve thermally evaporated blue PeLEDs with a peak external quantum efficiency (EQE) of 9.2 % and a maximum luminance of 1820 cd/m-- 2 (- |-), emitting at a peak wavelength of 488 nm. Furthermore, this strategy effectively suppresses ion migration, significantly enhancing the spectral stability and device lifetime of the blue PeLEDs. This work provides a process-compatible defect passivation strategy for thermally evaporated PeLEDs, laying the foundation for further advancements in this field.