For the possible application in high safety lithium ion batteries, nano-particle decorated poly(methyl methacrylate-acrylonitrile-ethyl acrylate) (P(MMA-AN-EA)) based gel polymer electrolyte (GPE) using 1-ethyl-3-methylimidazolium bis(trifluoromethanesolfonyl) imide (EMITFSI) ionic liquid as plasticizer was developed in this paper for the first time. Characterization of thermal stability indicated that the MS5 (SiO2: Al2O3 = 5:5 in weight) membrane presented better property of anti-thermal shrinkage, which in turn exhibited the excellent flame retardation performance for the corresponding GPE when incorporated with ionic liquid. The developed MS5 membrane had the strongest fracture strength of 160 MPa, much higher than that of polyethylene (PE) separator (138 MPa). Electrochemical analysis suggested that the GPE exhibited the enhanced ionic conductivity of 3.2 × 10−3 S cm−1 at ambient temperature, caused by its better porous structure with the highest porosity of 70%. The temperature dependence of ionic conductivity followed Vogel-Tamman-Fulcher (VTF) behavior. Oxidative potential of MS5 based GPE was significantly improved from 4.7 V (vs. Li+/Li) (PE saturated with ionic liquid) to 5.7 V. Theoretical calculation results showed that the stronger interaction between lithium ions and TFSI− anions was responsible for the better compatibility of GPE with the electrodes. Thus, MS5 based GPE exhibited excellent cyclic stability, which kept 95.1% capacity retention after 100 cycles under 0.2C rate in LiFePO4/GPE/Li type coin cell at room temperature, while retained 88.4% of initial discharge capacity at high temperature of 55 °C. Due to the enhanced transference number of lithium ion of the GPE, LiFePO4 cathode in LiFePO4/GPE/Li coin cell also presented superior rate performance.