Objective To design a kind of customized insole with zonal gradient hardness for people with high arch foot in need of plantar decompression. Methods A functional gradient structure was designed and applied to the customized insole. Porous elements with corresponding elastic modulus were used in different areas of insole. The relationship between structural element parameters and modulus was studied through mechanical tests. The foot geometry and plantar pressure distribution data of volunteers were collected, and the plantar region was divided according to the pressure contour line, so as to assemble the structural unit. Four kinds of customized insoles were designed: ordinary flat insole, optimized flat insole, ordinary full contact insole and optimized full contact insole. Through plantar pressure test experiment, the optimization design of sub region was verified. Results The designed insole could reduce the peak pressure of high arch foot by 52.8% in static standing state and 18.43% in gait condition. Conclusions This method can be used to design customized insoles, such as functional insoles for patients with diabetes and high arch feet, by providing better decompression function. The research findings provide references for conservative treatment of foot diseases with decompression needs.