The prime objective of the present paper is to develop polymer electrolyte doped with salt. A solution cast technique was adopted to develop a solid polymer electrolyte of polymer (Poly (methyl methacrylate) (PMMA) as host polymer and salt potassium hydroxide, KOH as dopant. Incorporation of salt increases the amophosity and homogeneity, decreases the surface roughness. Moreover, doping of salt (KOH) increases the overall conductivity of polymer electrolyte film. Electrochemical impedance spectroscopy (EIS) reveals the enhancement in conductivity (4 orders of magnitude) by salt doping. FTIR shows the complexation and composite nature of films. Polarized optical microscopy (POM) shows reduction in crystallinity which is further confirmed by Differential scanning calorimetry (DSC). Fabricated electrochemical double-layer supercapacitor using maximum conducting polymer salt electrolyte and symmetric carbon nanotubes electrodes shows specific capacitance of 21.86. Keywords:   Polymer electrolyte; conductivity; DSC; FTIR