The invention discloses a method for calculating heat efficiency of a boiler with a low-low-temperature coal saver-air heater running system. The method comprises the following steps of 1, calculating external heat Qwl of the low-low-temperature coal saver-air heater running system; 2, calculating exhaust gas heat loss Q2 by taking an environment temperature as an air supply temperature of an inlet and taking an outlet gas temperature of an air pre-heater as an exhaust gas temperature, and then obtaining converted exhaust gas heat loss Q'2; 3, performing reverse deduction calculation through the value of the converted exhaust gas heat loss Q'2 to obtain a converted exhaust gas temperature theta zs; and 4, calculating actually measured boiler heat efficiency by taking the converted exhaust gas temperature theta zs as an actually measured exhaust gas temperature and taking the environment temperature as the air supply temperature of the inlet, and correcting the converted exhaust gas temperature theta zs to a design air supply temperature of the inlet, thereby obtaining boiler heat efficiency subjected to air supply temperature correction.