The invention discloses an optimized ammonia injection method and system for a SCR (selective catalytic reduction) system of a thermal power plant. The method comprises the steps that a SCR (selectivecatalytic reduction) region of the thermal power plant is divided into two symmetrical sides, namely a side A and a side B, and purging is carried out on one side each time; in the 3 seconds before purging of a flue gas sampling device on the side A of the SCR (selective catalytic reduction) region, the opening degrees of ammonia injection adjusting valves on the side A and the side B of the SCR(selective catalytic reduction) region are detected, and the values of the opening degrees of the ammonia injection adjusting valves on the side A and the side B of the SCR (selective catalytic reduction) region are correspondingly recorded; the values of the opening degrees of the ammonia injection adjusting valves on the side A and the side B are compared; the opening degree of the ammonia injection adjusting valve during the purging period of the side A is determined according to the comparison result; and after purging of the flue gas sampling device on the side A is completed, the openingdegree of the ammonia injection adjusting valve on the side B of the SCR (selective catalytic reduction) region is adjusted according to the logic of the opening degree of the adjusting valve on theside B. The method has the advantages that the phenomenon that nitrogen oxide in flue gas exceeds the standard can be avoided, and the use amount of ammonia gas can be effectively reduced.