C₃N₆H₆·2H₃BO₃ (M·2B) is a highly promising precursor of boron nitride (BN) fibers due to its eco-friendly and low-cost fabrication. However, it is still unclear why the fibers can maintain their morphology in spite of drastic weight loss (nearly 80 wt%) during M·2B-to-BN pyrolysis. Herein, an interesting cracking and self-healing behavior of the heated M·2B fibers was observed at initial pyrolysis. In situ formed molten boron oxide (B₂O₃) was figured out to be the healing agent for the cracks and subsequently merged into the continuous matrix enclosing melamine/melem molecules, which subsequently acted as a nitrogen source. The B₂O₃ matrix helped to keep the fiber morphology undamaged under the second weight-loss stage in the pyrolysis process. This strategy of taking advantage of the in situ formed molten phase for healing cracks offers detailed guidance to prepare defect-free M·2B-derived BN fibers and would be significant in defect repair for other ceramics.