Solid polymer electrolytes are potentially useful electrolytes to be applied in high-energy batteries. In the present work, a novel polymer electrolyte, polyethylene oxide (PEO)–NaSCN–AlCl3, was prepared and investigated by FT-IR spectroscopic techniques. Based on the FT-IR data, the bands in the CN stretching envelope have been assigned and the effect of AlCl3 on ion–ion and ion–polymer interactions in the polymer electrolyte has been examined. It is shown that the Lewis acid–base interaction of AlCl3 with SCN1− leads to the formation of the complex anions AlCl3SCN− and Al2Cl6SCN−, depending on the content of AlCl3 and/or NaSCN in PEO; the preferential interactions of AlCl3 with crystal complex P(EO)3NaSCN occur in PEO–NaSCN–AlCl3 electrolytes; the AlCl3–NaSCN complex anions can play a plasticization role in PEO–NaSCN–AlCl3 electrolyte, and are expected to be a important factor to improve the conductivity and to enhance the cation transference number. In addition, the interactions between AlCl3 and ether oxygen of PEO were analyzed, and their effect on ionic association was also discussed.