Translation of the high mechanical properties of cellulose nanofibrils (CNFs) to macroscopic fibers represents a great challenge due to difficulties in the assembly of CNFs into well-ordered structures. In this study, we report the ultrastrong and flame-retardant microfibers via the microfluidic wet spinning of phosphorylated cellulose nanofibrils (PCNFs) with high charge content. The macroscopic stress is effectively transferred to the individual PCNFs and results in a Young's modulus of 29 GPa and a tensile strength of 654 MPa. The as-prepared microfibers retain >85 % strength in the wet hydration state, exceeding most natural or synthetic microfibers. Furthermore, glycerol and egg yolk were introduced to the microfibers for enhancing the modulus (31 GPa), strength (865 MPa) and the strain to failure (10.95 %). In addition, the PCNFs microfibers have good flame retardancy. This study expands the potential applications of nanocellulose microfibers in biomedical and flame-retardant materials.