The invention discloses a method for evaluating the influence of internal leakage of valves of a coal-fired power plant thermal system on units, comprising the following steps: determining the super-cooling degree of condensate of a condenser, and calculating the difference between the super-cooling degree value and a super-cooling degree value psi0 of condensate during acceptance test in the period of design or early production of the condenser; calculating internally-leaked heat; setting the internally-leaked heat as leaked steam heat, and calculating the amount of internal power converted from the steam heat; calculating the increase of unit power supply coal consumption rate caused by internal leakage according to the ratio of the amount of internal power to the generator power and the heat loss rate value under the amount of the internal power; and counting up the number of days d of internal leakage and the price gamma of each ton of standard coal, and calculating the economic benefit. The beneficial effects are as follows: the increase of unit power supply coal consumption rate caused by internal leakage can be calculated quantitatively, the influence of internal leakage on the economy of units can be evaluated quantitatively, and theoretical basis is provided for determining whether or not to perform valve modification on valves.