The crystallization of poly(ethylene oxide) (PEO) sodium iodine (NaI) composites is investigated by differential scanning calorimetry (DSC), extensional rheology, and in situ small-angle X-ray scattering (SAXS) with the aim of demonstrating versatile roles played by polymer ion interactions. In the 750 isothermal quiescent crystallization process, a decrease in the crystal growth rate is observed for PEO NaI and is attributed to slow chain movement caused by the coordination between cations and polymer. In situ SAXS on extensional flow induced crystallization (FIC) exhibits enhanced kinetics and orientation for both PEO and PEO NaI with increasing strain rate. However, an overall weaker strain-rate dependence of FIC is observed for PEO NaI, which can be interpreted as a synergistic consequence of promoted nucleation under flow and impeded crystal growth by polymer ion interaction. A possible microscopic mechanism is proposed to account for the experimental observation based on the formation of transient cross-linking points in PEO NaI and their influence on the entanglement network of polymer under various flow fields. The disclosed strain-rate dependence and various ion effects on the behavior of PEO salt composites contribute to a comprehensive understanding of polymer ion solid polyelectrolytes.