The invention provides a stepwise optimization method and a device for a monthly power generation plan. The initial planned power generation amount of a generator set is firstly determined; the shutdown days of the generator set are then determined; and a final planned power generation amount of the generator set is finally determined. Stepwise execution of pre-planning, shutdown planning and alternative trading planning at the beginning of a month, in the middle of the month and at the end of the month is carried out, which is closer to a planned business process; and due to the final plannedpower generation amount of the generator set obtained by the technical scheme provided in the invention, requirements that a clean energy source has the priority of power generation, the comprehensive cost is low, the power-on power generation load rate is in a normal level, and the workload for the alternative trading is small can be ensured, the obtained final planned power generation amount ofthe generator set is high in reliability, and security check can be passed.