Solid polymer electrolyte (SPE) films consisting of polyvinyl butyral (PVB) as host polymer, LiClO4 as alkali salt at mole ratio of [O]:[Li]=8, and different molecular weight polyethylene glycol (PEG) including PEG200, PEG400, and PEG600 as plasticizers are prepared by physical blending method. The dielectric relaxation and electrochemical impedance measurements reveal that the conductive performances are improved by adding PEG as plasticizers through the enhancement in the moving space for ions, and PEG400 performs plasticizing effect superior to PEG200 and PEG600. Their conductivity is measured by using a sandwiched Pt/SPE/Pt cell model. SPE with 30% PEG400 (wt%) of PVB exhibits the maximum conductivity at room temperature, and its conductivity increases linearly with temperatures from 303 to 333K at two to three orders of magnitude higher than that of the other two SPEs containing 30% PEG200 and 30% PEG600, respectively. However, their conductivity does not increase linearly with the increase in heating temperatures until the temperature reaches around 333K; the decrease in conductivity with heating from their maxima is attributed to the restriction of ion moving space because of the crosslinking reaction between hydroxyl and aldehyde groups. As observed from the XRD and the microscopy results, PEG400 is more effective than others in enhancing the conductive performances of these SPEs through changing LiClO4 from crystalline to amorphous state, increasing the flexibility of PVB, disturbing the short distance sequential order of PVB chains, and promoting the formation of ‘pathway’ for ions’ movement.