A method for measuring residual magnetic flux density of an iron core of a transformer comprises the following steps that (1) an initial magnetizing curve of the iron core of the transformer is obtained; (2) the direct current resistance of the transformer is measured, wherein high voltage direct current resistance and low voltage direct current resistance are measured respectively; (3) the exciting current of the tested transformer at a certain voltage is measured, the magnetic field intensity H of the iron core is determined and converted according to the exciting current, the magnetic flux intensity B of the iron core at the moment is obtained by searching the initial magnetizing curve; (4) the size of the alternating magnetic flux intensity B1 at the moment is calculated according to an applied voltage effective value, and the residual magnetism Br of the iron core at the moment is obtained simply by subtracting the alternating magnetic flux intensity B1 from the magnetic flux intensity B obtained by using the initial magnetizing curve. The method solves the problem that the residual magnetic flux intensity of the iron core of the transformer cannot be measured, the residual magnetic flux intensity of the iron core is determined by using a mathematic method, the method can be widely applied to the detection of the demagnetizing effect of demagnetizing equipment after demagnetization is carried out by the demagnetizing equipment, and fundamental significance is achieved on the safe and stable operation of the transformer.