The invention relates to the field of thermal control, and discloses a method for measuring the carbon content of fly ash. The method comprises the following steps: according to the historical data of carbon content of fly ash and auxiliary variables corresponding to the historical data, establishing a historical data sample assembly; picking a first sample assembly that is close to the current working conditions from the historical data sample assembly; according to the auxiliary variable values and fly ash carbon content values of the first sample assembly, fitting a weighting coefficient of the auxiliary variables; and finally calculating the fly ash carbon content under the current working conditions according to the auxiliary variable values and the weighting coefficient under the current working conditions.