Assembling polymer electrolytes (PEs) with lithium metal anodes is a promising strategy to address safety and specific capacity concerns. Molecular design on conductive polymer matrices avoids inherent high crystallinity, poor ion transport, and combustion drawbacks by adjusting the chain structure and functional groups. However, conventional structural modulation methods are difficult to conduct via in situ polymerization and cell assembly, thus hampering the further performance enhancement of PEs in lithium batteries. Herein, we report a simple aza-Michael addition method induced by lithium salts for designing and fabricating high-performance PEs. This method fulfills the in situ structure modulation of polyether-based PEs without any introduction of non-electrolytic components. The obtained polyether electrolytes exhibit a more amorphous cross-linked structure, leading to a favorable medium for ion migration and excellent flexibility. In addition, the introduction of C–N bonds imparts excellent non-flammability to the material as well as the ability to interact with sulfolane. Hence, the in situ-constructed gel polymer electrolyte (GPE) is characterized by a high ionic conductivity (1.76 × 10–4 S cm–1 at 30 °C) and excellent interface stability with the electrodes. The assembled Li/LiFePO4 (LFP) battery based on this newly designed GPE exhibits a high initial specific discharge capacity and a good rate performance (112.7 mAh g–1 at 5 C). Long-term stability was also demonstrated, with a capacity retention of 86.8% after 500 cycles at 0.5 C.