Abstract: With the aging of existing thermal power units and the continuous growth of social electricity load,it is urgent for thermal power generation companies to develop appropriate thermal power capacity expansion plan to meet the future load demand.In order to deal with this problem,a capacity expansion planning model for thermal power units based on double-layer optimization is established,with the investment cost and operation cost of thermal power units as the upper optimization objectives and the operation cost of the existing and future construction of thermal power units as the lower optimization objectives.Then,a solution method based on the primal duality theory is proposed.Finally,with a power plant as an example,programming simulation in Cplex verifies the validity and feasibility of the proposed double-layer model.