A method of harmonic tuning of a gong of a striking watch, the gong being fastened to at least one gong holder by at least one of its ends or by an intermediate portion located between the first end and the second end of the gong, and for which a ratio of vibration frequencies generated on the one hand in a watch plane XY, and on the other hand outside the watch plane according to the axis Z following a strike of a hammer against the gong, the method comprising the following steps: - placing the gong by means of its gong holder on an adapted support of a measuring instrument or placing a watch case comprising a striking mechanism with the gong on an adapted support of the measuring instrument, - striking the gong in a direction defined by a hammer which is external or forming part of the striking mechanism of the watch, - picking up the dynamic response of the gong activated and vibrated by the strike of the hammer, by the measuring instrument, the method being characterised in that it comprises the following steps: - processing the dynamic response signal of the gong by performing a fast Fourier transform in a processor unit or in a microcontroller of the measuring instrument, - determining at least two frequency peaks which correspond, in a selected natural mode, to the vibration frequencies in the plane XY and outside the plane Z of the vibrating gong in an audible frequency band from 20 Hz to 5 kHz, - calculating at least one ratio r = |fip - fih|/fip ≤ 0.006 or r' = |fih-fip|/fih ≤ 0.006, where fip is the vibration frequency in the plane XY of the i th selected natural mode, and fih is the vibration frequency outside the plane Z of the i th natural mode, the calculation of the ratio r or r' depending on the direction of striking of the hammer against the gong, - comparing the ratio r or r' to a desired value equal to 0.006 in such a way that if r or r' is equal to or less than the desired value of 0.006 the gong is considered tuned, on the other hand, if r or r' is greater than the desired value, a gong adjustment operation is carried out before repeating the steps of the method from the striking of the gong by the hammer. - placing the gong by means of its gong holder on an adapted support of a measuring instrument or placing a watch case comprising a striking mechanism with the gong on an adapted support of the measuring instrument, - striking the gong in a direction defined by a hammer which is external or forming part of the striking mechanism of the watch, - picking up the dynamic response of the gong activated and vibrated by the strike of the hammer, by the measuring instrument, - processing the dynamic response signal of the gong by performing a fast Fourier transform in a processor unit or in a microcontroller of the measuring instrument, - determining at least two frequency peaks which correspond, in a selected natural mode, to the vibration frequencies in the plane XY and outside the plane Z of the vibrating gong in an audible frequency band from 20 Hz to 5 kHz, - calculating at least one ratio r = |fip - fih|/fip ≤ 0.006 or r' = |fih-fip|/fih ≤ 0.006, where fip is the vibration frequency in the plane XY of the i th selected natural mode, and fih is the vibration frequency outside the plane Z of the i th natural mode, the calculation of the ratio r or r' depending on the direction of striking of the hammer against the gong, - comparing the ratio r or r' to a desired value equal to 0.006 in such a way that if r or r' is equal to or less than the desired value of 0.006 the gong is considered tuned, on the other hand, if r or r' is greater than the desired value, a gong adjustment operation is carried out before repeating the steps of the method from the striking of the gong by the hammer.