An excellent silicon/phosphorus double-effect flame retardant, prepared by modifying 3-glycidoxy-propyltrimethoxysilane (KH560) with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), was compounded with polyethylene oxide (PEO) and lithium salts to prepare polymer solid electrolytes, and its influence on the thermal stability and electrochemical performance of PEO based electrolytes was studied. It is found that the conductivity of the DOPO-KH560/PEO/LiTFSI electrolyte is as high as 1.8×10^-4 S/cm when the addition amount of DOPO-KH560 is 10% and n(EO)∶n(Li⁺) =8∶1 and it can maintain the electrochemical stability below 4.9 V. The LiFePO₄/Li half battery prepared with the electrolyte can be used stably at 80 ℃, and deliver a reversible specific capacity of 136.5 mA·h/g at 0.2 C after 80 cycles, showing an excellent cycle stability.