The invention discloses a multi-day unit combination optimization method and system. The method comprises the steps of setting an optimization period and an optimization time period of multi-day unitcombination optimization calculation; substituting the optimization period and the optimization time period into a multi-day unit combination optimization model, and solving by adopting a mixed integer programming method to obtain start-stop time and output arrangement of each unit in the optimization period; taking the start-stop time and the output arrangement of each unit in the optimization period as the basis of day-ahead market clearing calculation; wherein the units comprise a coal-fired unit, a gas unit, a new energy unit, a nuclear power unit and a hydroelectric unit; wherein the coal-fired unit comprises a newly-added coal-fired unit which can be started only by notifying in advance. The invention is suitable for calculation of multi-day unit combination modes of different typesof days in a spot market environment, resources can be coordinated and optimized for multiple days in advance under the condition that the coal and gas quantities are insufficient or incoming water isinsufficient, and the problem that a coal-fired unit cannot be newly added in day-ahead market clearing calculation is solved.