The invention provides a transformer reactive power compensation method and device. The method comprises the steps that the voltage and the current of the direct-current end and the voltage and the current of the alternating-current end of a converter transformer are detected, the phase difference between the voltage of the alternating-current end and the current of the alternating-current end is calculated, and the phase difference between the voltage of the direct-current end and the current of the direct-current end is calculated; the power of the alternating-current end is calculated by using the phase difference between the voltage of the alternating-current end and the current of the alternating-current, the voltage of the altering-current end and the current of the alternating-current end as parameters; the difference between the power of the alternating-current end and the power of the direct-current end serves as reactive power compensation power to be output. According to the transformer reactive power compensation method and device, the direct-current side electric power and the alternating-current side electric power of the converter transformer are detected and calculated, the difference between the alternating-current end power and the direct-current end power is obtained and serves as the reactive power compensation power needed by reactive power compensation, reactive power compensation can be carried out on a power supply network, and thus corresponding reactive power compensation can be carried out on related devices of direct-current power supply.