The combination of polymers with ionic liquids (ILs) is of significant interest for a variety of possible applications e.g., polymer electrolytes, nanoencapsulations and actuators. Knowledge of phase separation, crystallization and their interplay in polymer/IL mixtures is of crucial importance for realizing these applications. In this work, the effect of phase separation on crystallization behavior for poly(ethylene oxide)/1-ethyl-3-methylimidazolium tetrofluoroborate (PEO/[EMIM][BF4]) mixtures with a lower critical solution temperature (LCST) was investigated for the first time. It is demonstrated that the crystallization of PEO/ILs mixtures could be highly accelerated by phase separation as the blended samples are quenched from two-phase region compared to those quenched from homogenous state. This phenomenon can be on one hand ascribed to the disruption of hydrogen bonds between PEO and IL molecules as evidenced by FTIR results, which causes an increase of chain mobility and diffusivity for developing a spherulite embryo, and on the other hand due to fluctuation concentration where local PEO-rich domains with higher crystallizable PEO concentration will exist, even though the interfacial area has disappeared upon quenching to the crystallization. The facilitating effect of phase separation on crystallization becomes more prominent as driven by spinodal decomposition (50 wt% PEO) than that driven by nucleation & growth (70 wt% PEO). This work will provide a new possibility to optimize the performance of polymer electrolytes by regulating the phase morphologies of polymer/IL blends.