<p>The highly spontaneous burning of magnesium alloys severely hinders their safe production and application. However, a clear understanding of the burning mechanism has been lacking. The present work in situ investigates the burning behavior of EV31 magnesium alloys with neodymium addition. Ignition modes, starting from the fully molten state, were observed, in contrast to the proposed solid-state ignition and microscale pre-melt pool ignition modes. The ignition was triggered by the extremely rapid oxidation of the melt leaking from cracks in the surface oxide shell at temperatures much higher than the melting point. Fresh cracks and new oxide films formed at these cracks were observed on the burning sample’s surface upon in situ fire extinguishing, providing strong evidence to support the hypothesis that ignition is initiated by cracks in the surface oxide film. Neodymium addition reduced the cracking of the surface oxide shell and increased the ignition temperature in a linear relationship within the currently investigated content range 3−15 wt% of Nd. This study reveals an unexpected ignition mode of magnesium alloys and figures out the microscopic effects of neodymium on the flame resistance of these alloys, which could inform the development of novel flame-resistant magnesium alloy technologies.</p>