Cellulose separators present promising properties applied in lithium-ion batteries but are confronted with strength issue in electrolyte. In this paper, mechanical and electrochemical properties of lithium-ion battery cellulose separator under electrolyte-immersed condition are studied. The tensile properties of the cellulose separators concerning electrolyte immersion time are investigated at multi-scales, which are fiber components, single fiber and cellulose separator. Electrochemical properties of cellulose separator are also studied. Experiments upon polypropylene/PP separator are also conducted for comparison. The results reveal that strength weakening and swelling both occur in fiber components, single fiber and cellulose separator. Softening of fiber components directly results in softening of the separator. Reduction of Young's modulus and tensile strength mainly happens within 30 min of immersion, while growing of elongation rate sustains as time proceeds. Swelling is verified to attribute to improvement of the electrochemical properties of cellulose separators, in which case, smaller interfacial impedance and larger electrochemical stable window were found. Compared to PP separator, the strength weakening of cellulose separator are relatively serious, which suggesting that balance should be explored between mechanical properties and electrochemical performance for cellulose separator.