Provided is a method for testing the interface state of a three-port silicon carbide based power device. Through the method, the average interface state density at the interface of a junction field effect region and a gate oxide layer of a three-port silicon carbide based power device can be extracted easily and quickly. The gate of the three-port silicon carbide based power device is externally connected with a pulse voltage with fixed frequency and amplitude and changing base voltage V0 or a pulse voltage with fixed frequency and base voltage V0 and changing amplitude Vp, the source is externally connected with the negative electrode of a reversed bias voltage, the drain is externally connected with the positive electrode of the reversed bias voltage, the drain is externally connected with the detection current of an ammeter, and the ammeter is connected in series to a loop composed of the drain, a voltage source and the source, thus obtaining a current and voltage curve. The averagedistribution of the interface state above the junction field effect region can be calculated through a formula according to the peak current of the current and voltage curve, and the degradation of the interface between the junction field effect region and the gate oxide layer above the channel region of the device under stress can be judged by comparing the test curves before and after stress.