Abstract: An infill topology optimization method based on a hybrid variable strategy is proposed for the lightweight design of porous structures under thermo-mechanical loads. Traditional topology optimization methods often face challenges in the topology optimization of thermoelastic structures, including design-dependent loads with complex sensitivity analysis, high computational costs, and gray regions in the optimization design results that blur structural boundaries and hinder manufacturability. To address these issues, this paper proposes a novel infill topology optimization method that combines continuous and discrete variables, specifically for topological optimization design of porous structures under thermo-mechanical loads. By replacing global volume constraints with local volume constraints, the local material density distribution is effectively controlled, avoiding excessive uniformity in the generated porous structures. The design optimization first performs continuous-variable topology optimization on a coarse mesh, followed by discrete-variable infill design on a refined mesh. The present local volume constraints not only facilitate the formation of substructures and guide the optimization process toward achieving an uniform material distribution, but also provide physical significance to the design variables in continuous variable topology optimization design through the connection between continuous and discrete variable methods, preventing a significant increase in the objective function value when the design optimization results are overly uniform. Furthermore, an improved sensitivity filtering strategy is developed to achieve superior topological designs with enhanced objective function values. The numerical examples on a bi-clamped beam model demonstrate the effectiveness of the present method and the effects of filter radius and temperature difference on optimal designs are investigated. It is shown that the proposed optimization method successfully converts continuous variable optimization design results under coarse mesh into discrete variable porous structure designs under fine mesh with clear geometric features to significantly enhance the manufacturability of topological design, while the optimized structures exhibit excellent thermal distortion resistance and mechanical performance.