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Lithium dendrites (filaments) propagation in the solid electrolyte (SEs) leading to short circuits is one of the biggest obstacles to the application of all-solid-state lithium metal batteries (ASSLMBs). Due to the lacking of operando techniques that can provide nano-scale spatial resolution, the insufficient knowledge of the lithium dendrite growth inside SEs makes it difficult to suppress dendrite formation. To reveal the mechanism of Li filament growth in SEs, we achieved real-time monitoring of the nanoscale Li filament growth by operando small-angle neutron scattering (SANS) in a Cu|LLZNO|Li cell. On continuous plating, Li filament growth is not simply an accumulation of Li, but there is a dynamic evolution due to the competition between Li filament formation and self-healing. This dynamic evolution of Li filaments has been proved to be prevalent in typical SEs. With the aid of simulations and experiments, this dynamic competition was demonstrated to be highly dependent on the temperature variation. The enhanced self-healing ability of Li at elevated temperatures plays a positive role in suppressing Li filament formation. The cycle life of the cell was improved by heat therapy, which provided a brand-new insight for suppressing Li filament formation. Operando SANS with high Li sensitivity and high spatial resolution provides a new platform for investigating Li filaments in SEs.