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The separator, as an important inner part of the sodium‐ion battery (SIB), has a significant impact on the electrochemical performance and security of the battery. However, conventional polyolefin separators are inapplicable for SIBs due to their poor wettability to liquid electrolytes and unsatisfactory heat resistance. To address these problems, a novel polyethylene (PE)‐ hydroxyethyl cellulose (HEC)‐TiO2composite separator modified on the PE matrix is proposed and successfully prepared by a multistep synthesis procedure of HEC coating and TiO2in situ self‐growth, while almost maintaining the initial separator thickness. Compared with conventional PE separators, this composite separator possesses remarkable wettability which benefits from the introduction of a polar HEC‐TiO2‐incorporated coating. Besides, thanks to a significant improvement in wettability, the separator presents high electrolyte uptake of up to 186.5% and an extraordinary ionic conductivity of 0.342 mS cm−1. As expected, a Na|Na3V2(PO4)3battery with the PE‐HEC‐TiO2separator exhibits a reversible capacity of 99.0 mAh g−1and a capacity retention of 94.8% after 1000 cycles at 5 C with a steady Coulombic efficiency of nearly 100%. These brilliant performances convincingly make it a promising separator for advanced SIBs with high reversibility, high capacity, and long life. A novel polyethylene (PE)‐ hydroxyethyl cellulose (HEC)‐TiO2composite separator modified on the polyethylene matrix is proposed and successfully prepared by a multistep synthesis procedure of hydroxyethyl cellulose coating and TiO2in situ self‐growth. The modified separator possesses excellent thermal stability, remarkable wettability, and high ionic conductivity, resulting in superior long cycle life for the cells equipped with it.