The invention relates to a gas turbine set waste heat boiler flue gas flow calculation method. The method comprises the steps that firstly, a cross section grid method is used for arranging measuringpoints at a waste heat boiler smoke exhaust outlet; then measuring the smoke components and content of each measuring point position, and calculating the smoke density; measuring the temperature, thedynamic pressure and the static pressure of the flue gas at each measuring point position, and calculating the flow velocity of the flue gas; final calculation of flue gas flow. Only flue gas dynamicand static pressure values, temperature, flue gas components and content, local atmospheric pressure and cross sectional area of a flue at a measuring point position need to be measured on site; measurement data are relatively few, the operation state of the waste heat boiler can be visually reflected, the data have high reliability, the calculation process is convenient and fast, and the method can be verified with a gas turbine heat balance calculation model and a waste heat boiler heat balance calculation model.