Abstract The electroforming process, transforming a homogeneous insulating oxide into localized conductive filaments, is crucial for memristive devices. However, it is still unclear how the intermediate phases develop microscopically throughout the transient forming process. Here, we investigate the nonequilibrium dynamic phase transition in the conductive region of TiO₂ memristors during electroforming. Synchronous electroluminescence emission and transport measurements demonstrate that the application of pulse fields primarily causes a gradual reduction in the conducting area, accompanied by the reversible field-dependent evolution of metastable phases at the cathode region. As a result of positive feedback between the lateral gradient of oxygen vacancies and the electric field, the self-reinforcing process eventually facilitates the final filament generation. This study offers insights into the physical mechanisms governing the metastable phase evolution during electroforming and raises implications for optimizing the forming process of memristive devices.