With their unique combination of low density, high thermal conductivity, high thermal stability, excellent chemical inertness, and good dielectric properties, boron nitride (BN) fibers are promising for advanced material applications, particularly in the field of electromagnetic wave transmission. The fabrication of high-quality, continuous BN fibers via inorganic methods still faces unresolved challenges. In this study, continuous boron oxide (B2O3) precursor fibers were initially produced using a melt spinning process with boric acid as the raw material. Subsequently, primary BN fiber products containing residual B2O3 (referred to as BNO) and final BN fibers were successively prepared through a two-step nitridation process. The effects of the drawing process and the initial nitridation temperature on the composition and phase constitution of B2O3 were extensively investigated. Additionally, it explored the processes of crystallization and orientation evolution in continuous BN fibers, thereby elucidating the impact of micro-structure on the mechanical properties of both BNO and BN fibers. It is noteworthy that the continuous BN fibers, subjected to a secondary nitridation process at 1850°C, exhibited exceptional mechanical properties, with a tensile strength of 890.9 MPa and an elastic modulus of 40.8 GPa. This study not only produce continuous BN fibers with exceptional mechanical properties but also provide a detailed description of their synthesis process and structural evolution, offering valuable insights for enhancing the mechanical performance of continuous fibrous materials with two-dimensional structure.