The gel polymer electrolyte (GPE) is a promising substitution for traditional liquid electrolytes. However, GPE is still troubled mainly by its sluggish ionic conductivity and inferior interfacial compatibility with electrodes. Herein, a phosphorus-modified GPE was fabricated by in situ incorporation of black phosphorus (BP) nanosheets into a poly(methyl methacrylate) (PMMA) matrix during the self-polymerization of monomers. The developed GPE exhibited high ionic conductivity (1.083 mS·cm–1 at 30 °C), an enhanced Li+ transference number (0.43), and a wide electrochemical stability window (5.2 V vs Li+/Li), while good thermal stability and improved flame retardancy can also be achieved. Differential scanning calorimeter measurements confirmed that the crystallinity of the PMMA matrix was not changed as BP nanosheets were incorporated. Further investigation proved that BP nanosheets contained in PMMA segments effectively immobilized the anions to decrease the coordination number around Li+. As a result, Li+ ion transport through the GPE was facilitated, which promoted the uniform stripping/plating of lithium while cycling the lithium symmetry cell. Based on the phosphorus-modified GPE, the Li|LiFePO4 and Li|LiNi0.5Co0.2Mn0.3O2 batteries and graphite|LiFePO4 soft-package battery exhibited encouraging electrochemical performances and safety properties.