In this work, a series of polyimide fibers (PI) based on 3,3,4,4-biphenyltetracarboxylic anhydride (BPDA)/p-phenylenediamine (PDA) were prepared through a partial pre-imidization process, and the properties of these fibers were investigated in detail. The pre-imidization degree (pre-ID) was successfully controlled by adjusting the amount of dehydration reagents, which was confirmed by Fourier Transform Infrared. The scanning electron microscope images show that the addition of dehydration reagents in the poly(amic acid) spinning dope improves the shaping of the precursor fibers, which is related to the thermodynamic phase separation in the coagulation bath. The WAXD patterns indicate that the formed partial rigid-rod and oriented PI chains in the pre-imidization process are beneficial for forming a more ordered and regular crystalline structure in the post-heat-drawing process, thus improving their mechanical properties, which exhibit an optimum tensile strength and modulus of 1.7 GPa and 95.0 GPa, respectively. Meanwhile, the prepared PI fibers exhibit excellent thermal stabilities with the 5wt% (T-5%) weight loss temperature ranging from 530 to 568 degrees C depending on the pre-imidization degree. This work provides a simple and facile approach to preparing high-performance PI fibers through the partial pre-imidization process.