A kind of organic-inorganic nanocomposite of methacrylisobutyl-polyhedral oligomeric silsesquioxane (MA-POSS) is synthesized and the nanocomposite as the modified filler is introduced into polymethyl methacrylate (PMMA) matrix to prepare gel polymer electrolyte (GPE) membrane used in lithium ion battery (LIB) through phase inversion method combined with liquid electrolyte adsorption. The results reflect that compared to the solvent evaporation technique, the electrochemical performances of the GPE corresponding to the membrane prepared by phase inversion method are more outstanding. Besides, the membrane also shows good mechanical strength. Compared to the GPE based on pure PMMA matrix, the comprehensive performances of the modified GPE are also enhanced a lot and the GPE with the mass of 10% MA-POSS exhibits the most favorable properties. The excellent performances are shown as follows: the ionic conductivity reaches to 2.77 × 10−3 S cm−1 at room temperature; the lithium ion transfer number is up to 0.40; the GPE is electrochemical stable below 5.5 V (vs. Li/Li+); the compatibility between GPE and lithium anode is satisfying and the ionic conductivity obeys the Arrhenius behavior.