The invention provides a large-current synthesis transient state circuit for an electronic current transformer. The large-current synthesis transient state circuit comprises a controller unit, a direct-current charging power supply and a waveform output unit, the waveform output unit is connected with an ECT test product, the controller unit comprises a charging control module, a discharging control module, a waveform monitoring module and a transient state waveform adjustment module, the controller unit is used for controlling the work state of an electronic component in the waveform output unit, and accordingly the waveform output unit is controlled to output transient state voltage waveforms and transient state current waveforms to the ECT test product. Compared with the prior art, the large-current synthesis transient state circuit for the electronic current transformer can accurately adjust current peak values and attenuation time constants of the output transient state voltage waveforms and the output transient state current waveforms, so that detection requirements of electronic current transformer test products of different models are met.