This presentation we report a new strategy to demonstrate a very stable high EL brightness by using our one-step solution processing method to prepare high quality perovskite (MAPbBr3) films with mixed large/small grains in micrometers/nanometers. Essentially, our design of using mixed large/small grains presents a self-passivation mechanism of grain boundary defects through ion migration under EL operation, consequently leading to a high brightness of 21,233 cd/m2 at the current density of 562 mA/cm2 in our device without current efficiency roll-off even at current densities. On the other hand, the self-passivation of grain boundary defects allows the turn-on voltage (1.9 V) lower than the bandgap (2.25 eV) to operate EL actions. The self-passivation of grain boundary defects is verified by the slow relaxation of both spectral intensity and shift when the photoluminescence (PL) is modulated by an electrical injection in our perovskite LEDs. Specifically, we observe that the PL intensity is slowly relaxed in the order of ~ 1 minutes after removing electrical injection while constant photoexcitation is applied. Simultaneously, the spectrum is blue shifted by ~ 15 nm due to the de-passivation upon removing electrical injection in the mixed large/small grains. Therefore, using mixed large/small grains with one-step solution processing method presents an important strategy to develop high-performance perovskite light-emitting devices based on self-passivation of mechanism of grain boundary defects.