A conceptual Enhanced Geothermal System (EGS) with triplet horizontal well layout is proposed, in which the injection well is flanked by two production wells. The injection well is completed with perforated casing, followed by multistage fracturing with casing packers. Two production wells are drilled through the stimulation zone induced by multistage fracturing and completed openhole to maximize contact with the reservoir. Based on a fully coupled hydrothermal (HT) model accounting for local thermal non-equilibrium (LTNE), sensitivity analysis and optimization for EGS design parameters are conducted. Simulation results show that increasing circulation rate only facilitates the heat extraction rate before thermal breakthrough, but cannot improve the heat extraction performance at the end of EGS operation. Compared with the fracture aperture, the increase in the stimulation zone permeability is more conducive to reducing the injection pressure and increasing the cumulative output thermal power. Insufficient or excessive fracturing stages are detrimental to economic heat extraction. It is found that more uniform flow into each perforation can achieve better thermal sweep efficiency and prevent local thermal breakthrough. The stimulation zone has an optimal reservoir permeability to match the fracture aperture. The optimum number of fracturing segments for a specified wellbore length is obtained. The preferable zone for the production well is between the fracture front and the outermost periphery of the stimulation zone.