KAg4I5(10%AgI) composite is prepared by the solid-state reaction method in the dark and dry conditions, and its structure, morphology, ion conductivity properties, and phase transition temperature are studied by X-ray diffraction, scanning electron microscope, impedance spectroscopy, differential scanning calorimetry and other analytical tools. The results show that when the two phases (AgI and KAg4I5 phases) in the composite are both fast ionic conductor phases, ionic conductivity of composite is higher than that of the single phase, and the heating and cooling of the conductivity curves form a hysteresis loop.During heating and cooling, AgI phase transition temperature lags 5 and 10 ℃ respectively.We use the interaction interface, the interface stress phase and Gouy-Chapman model to analyze the mechanism for the improved conductivity of this composite and phase transition temperature change when the two phases are both the fast ionic conductive.