This study reports the synthesis of a gel polymer electrolyte (GPE) with enhanced ion transport characteristic, based on the composite polymer of poly(methyl methacrylate) (PMMA) and succinonitrile (SN). Moreover, to demonstrate the practical applications, the GPE was applied to the assembly of the electrochromic devices (ECDs) with both glass and plastic substrates. In this composite polymer electrolyte, PMMA served as the polymer matrix and propylene carbonate (PC) was used as the plasticizer, which also provided better dispersion of the plastic crystal SN within PMMA. The composition ratio of PC to SN was investigated by measuring the ionic conductivity of the GPEs. The optimized ratio of PC to SN was 4:1 with an ionic conductivity of 1.46 mS center dot cm(-1). For the fabrication of the ECDs, tungsten oxide (WO3) was used as the cathodic coloring film, which is fabricated by pulsed de magnetron reactive sputtering. Prussian Blue nanoparticles (PBNPs) were synthesized and coated on the transparent Sn-doped indium oxide (ITO) substrates for the anodic coloring material. The performance of the 5x5 cm(2) WO3-PBNPs ECD was confirmed by observing its spectroelectrochemical behaviors. This quasi-solid-state ECD fabricated with the PMMA-SN composite polymer showed an optical contrast of 52.4% at 695 nm. The optical transmittance of the ECD could be reversibly modulated from 57.9% (bleached) to 5.5% (darkened) at 695 nm, by applying potentials of 1.8 and -2.0 V, respectively. During the durability test, the transmittance change (Delta T) of this ECD remained 44.5% after 2250 cycles, which was 85% of its original.