Polymer electrolyte has been attracted ever-increasing attentions in the field of in all-solid-state lithium-ion batteries. But the low ionic conductivity at ambient temperature and poor mechanical strength hinder its broad applications. Based on the extensive literature reading and fundamental experiments, we have proposed Methyl 2-hydroxyethyl cellulose(HEMC) as the organic filler to enhance PEO-based polymer electrolyte. Thus we have obtained a kind of flexible and superior all-solid-state polymer electrolyte. Herein, we discuss the enhancement on ionic conductivity reinforced by active inorganic fillers-Li6.4La3Zr1.4Ta0.6O12 (LLZTO). The obtained flexible and all-solid-state polymer organic/inorganic composite polymer electrolyte with high conductivity at ambient temperature, good mechanical strength, wide electrochemical window and super stability to anode lithium. Also, we have investigated the fundamental mechanism of organic/inorganic composite polymer electrolyte. There is a threshold in the process of reinforcing polymer electrolyte on ionic conductivity by active inorganic fillers. The conductivity is dominated by polymer base when it’s below the optimal value, and vice versa. It provides distinctive solutions and illumination for designing and fabrication of practical all-solid-state electrolyte.