The present invention provides a transformer short-circuit impedance on-line monitoring method and device. The method comprises: collecting the current signals and the voltage signals of the high-voltage side and the low-voltage side of the transformer; determining the fundamental wave vector values of the high-voltage side and the low-voltage side of the transformer according to the collected current signals and the voltage signals of the high-voltage side and the low-voltage side of the transformer; sending the determined fundamental wave vector values of the high-voltage side and the low-voltage side of the transformer to an upper computer; and determining the short-circuit impedance value of the transformer through the upper computer according to the fundamental wave vector values of the high-voltage side and the low-voltage side of the transformer. therefore, the transformer is not needed to quit the operation, the online monitoring of the short-circuit impedance of the transformer. Because the electric signal collection device sends the determined fundamental wave vector values of the high-voltage side and the low-voltage side of the transformer to the upper computer, the method and device provided by the invention can save the communication time, have the timeliness, and can actually and accurately monitor the short-circuit impedance condition of the transformer. The present invention further provides a transformer short-circuit impedance on-line monitoring method and device having the same effect.