Boron nitride (BN) fiber is an important reinforcement for the preparation of ultra-high temperature ceramic-based wave-transparent composites, and it is a key material for the development and application of high Mach number aircraft wave-transparent radomes. Therefore, through investigation into the relationship between the microstructure and mechanical properties of BN fibers is particularly important for the progress of new-generation equipment. In this study, BN precursor fibers were prepared using the melt spinning method, and the structure and properties of BN fibers during the inorganic and ceramicization heat treatment were systematically characterized. The BN precursor fibers were converted into inorganic amorphous BN intermediate fibers, which were subsequently sintered at high temperatures to yield microcrystalline BN fibers, ultimately forming a structure composed exclusively of B—N—B hexagons. The study investigated the alterations in microstructure, structural transformation, crystallinity, and mechanical properties of the fibers during heat treatment under N2 and NH3 atmospheres. The results indicat that when NH3 is employed as the reaction atmosphere, the intermediate fibers have a dense structure, containing BN microcrystals (t-BN) with distinct lattice fringes, and the carbon content is only 0.09% (mass fraction). The tensile strength and elastic modulus of BN-10NH-16 fibers ceramicization reaches 832.14 MPa and 74.88 GPa, respectively. When N2 is employed as the reaction atmosphere, the cross-section of the BN intermediate fibers exhibit obvious pores and defects, but no microcrystals are formed, and the carbon content remaines at 18.00% (mass fraction). The tensile strength and elastic modulus of the BN-10N-16 fibers after ceramicization reaches 383.98 MPa and 71.73 GPa, respectively. XRD and HRTEM results indicate that the crystallinity of inorganic BN-10NH-16 fibers are significantly improved at 1 600 ℃ under NH3 atmosphere, and there is a certain degree of orientation along the axis direction of fibers, which thereby significantly enhances the mechanical properties of the fibers. Additionally, the relationship between microstructural defects and the mechanical properties of the fibers is verified by Weibull distribution fitting, providing an important reference for the subsequent preparation of high-performance BN fibers.