In situ formation of gel polymer electrolytes (GPE) has been a promising candidate to address individual limitations of liquid/solid electrolytes and interfacial stability. However, the controllable conversion of liquid electrolyte (LE) precursor to GPE remains a great challenge with lower lithium-ion transport, which is far from the demand for fast-charging properties. Herein, a strategy of gradient polymerization of forming GPE is pioneered, stabilizing the electrolyte/electrode interface with an accelerated Li+ migration feature. As demonstrated by theoretical simulations and visualization experiment results, the formation mechanism of GPE via a partial inhibitory mechanism of Lithium nitrate (LiNO3) to control the solvent polymerization is comprehensively investigated, exhibiting the preferential interaction between nitrate anion (NO3−) and the Lewis acidic site in lithium bis(fluorosulfonyl)imide (LiFSI). Consequently, a stable amorphous GPE with high Li+ conductivity (5.10 mS cm−1) and an inorganic solid electrolyte interphase (SEI)-dominate layer derived from spectroscopical measurements are achieved on the graphite electrode surface. The as-prepared lithium iron phosphate (LFP)||graphite pouch cell stabilizes the capacity of 109.80 mAh g−1 (capacity retention: 80.02%) after 715 cycles at 5 C/1 C (charge/discharge), corresponding to the energy density of 277.64 Wh kg−1. This work provides a facile but practical approach to designing a highly stable GPE for fast-charging lithium-ion batteries.