The invention discloses a calculation method suitable for NOx emission of an ultra-supercritical boiler and relates to a running control method for the ultra-supercritical boiler. The calculation method comprises the steps of 1) dividing a combustion chamber into a plurality of small chambers from bottom to top according to a combustion chamber structure and calculation precision; 2) inputting feed parameters; 3) calculating a reaction rate of each material in each small chamber; 4) calculating radiative heat transfer capacity of flame and flue gas in each small chamber for a water-cooled wall and radiative heat transfer capacity of the adjacent small chambers; 5) establishing mass and energy balance equations of each small chamber; 6) solving a conservation equation and outputting data of NOx generation capacity of each small chamber under current operational condition; and 7) changing the feed parameters and repeating the steps 2) to 6) to obtain data of NOx generation capacity of each small chamber under other operational conditions. According to the calculation method, the NOx emission of the ultra-supercritical boiler can be analyzed and calculated; the influence of the feed parameters on the NOx emission in the boiler can be visually reflected; and a calculation result can be used for guiding optimization control of the NOx emission of the ultra-supercritical boiler in actual industrial operation.