The invention provides a method for determining unit power generation efficiency. The method comprises the following steps: obtaining relevant heat information which is previously supplied to a boiler by an air preheater; according to the relevant heat information, obtaining boiler efficiency; and through the boiler efficiency and the heat consumption amount of a steam turbine, obtaining the power generation efficiency of a unit. Therefore, the boiler efficiency and the power generation efficiency of the unit with external supply heat can be simply and precisely calculated.