The utility model provides a transient waveform circuit of electronic current transformer heavy current synthesis, comprising a direct current charging power supply and a waveform output unit. The waveform output unit comprises a charging thyristor, a double-throw switch, a discharge thyristor, an adjustable resistor and an adjustable resistor which are successively in connection with an anode end of the direct current charging power supply, and a low sensitivity diverter in connection with a cathode end of the direct current charging power supply. Compared with prior art, the transient waveform circuit can perform precise adjustment on current peak values and fall time constants of outputted transient voltage waveforms and transient current waveforms to meet test requirements of electronic current transformer samples of different models.