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To provide molecular‐scale insight into the structural evolution from tris(methylamino)borane to boron nitride (BN) fiber during the chemical thermal‐treating process, polymeric green fiber is cured in hot synthetic air at 300°C and then treats to 400, 600, 800, and 1000°C in ammonia. The chemical composition and structure of the volatile compounds and residual products are analyzed during the pyrolysis process for the polymeric green fiber. It is demonstrated that oxygen can be used to cure polymeric green fiber rapidly under the premise of ensuring a final fiber content of less than 1 wt% carbon and 2 wt% oxygen while maintaining the fiber tensile strength at 1000°C. The molecular structure evolution during the pyrolysis process for polymeric green fiber after oxygen curing is determined. Specifically, in hot synthetic air, introducing oxygen and releasing methylamine generates a B–O six‐membered ring structure in the polymer in the first stage. Then, the removal of methyl results in the formation of a B–N–O network in hot ammonia. Afterward, nitridation of the B–O six‐membered ring promotes the evolution of the B–N six‐membered ring structure with the release of water and carbon dioxide. Finally, the growth and rearrangement of the BN structure are achieved. [ABSTRACT FROM AUTHOR]