A heuristic search method for the monthly thermal power unit combination problem of a power system relates to the field of power generation dispatching of a power system. At present, when the unit scale is large, it is easy to fall into the curse of dimensionality, the search speed is slow, and it may lead to non-unique solutions, which all can hardly meet the need of fast developing power grid dispatching operation in our country. The technical scheme adopted is as follows: an index, namely, 'subsequent load rate', is introduced to describe 'open, fair and just' power dispatching, and the mode of unit combination is optimized with the goal of 'equal subsequent load rate' of power stations. In the process of optimization, the subsequent load rate of power stations and the system reserve capacity margin are used as heuristic information, the start-up mode of the power stations is iteratively revised, a unit start-stop strategy in which large units consuming less energy are started first or small units consuming more energy are stopped first is adopted, and therefore, energy-saving power generation of the grid is realized. Through the technical scheme, the principle of 'open, fair and just' power dispatching can be satisfied, the fairness of power generation among power stations is ensured, and the demand of energy-saving power generation is taken into account. The heuristic search method is practical and feasible.