The invention provides a secondary air door openness optimizing method and a system. The method comprises the following steps: the overheating desuperheating water flow, the reheating desuperheating water flow, the concentration of carbon monoxide in hearth outlet smoke, the fly ash carbon content and the value of NOx concentration in denitration inlet smoke converted under the set oxygen quantity under each preset openness of each layer of secondary air doors of a boiler are measured; the unit operation cost variation under each preset openness of each layer of secondary air doors of the boiler is determined according to the overheating desuperheating water flow, the reheating desuperheating water flow, the concentration of carbon monoxide in hearth outlet smoke, the fly ash carbon content and the value of NOx concentration in denitration inlet smoke converted under the set oxygen quantity under each preset openness of each layer of secondary air doors of the boiler; and the openness of each layer of secondary air doors of the boiler corresponding to the minimum unit operation cost variation of each layer of secondary air doors is determined in the unit operation cost variation under each preset openness of each layer of secondary air doors of the boiler. The method can guarantee the unit operation economy.