The invention discloses an online transient test system for an electronic current transformer. The system comprises a standard source transient signal acquisition system, a detected electronic current transformer, a detected transformer transient signal acquisition system and a transient signal processing system, wherein the standard source transient signal acquisition system is arranged at a position close to a primary side system of a detected transformer. A transient large current of the primary system is accurately measured, the current is made to pass through the standard source transient signal acquisition system and then the detected electronic current transformer, transient signals passing through the two are respectively received by the transient signal processing system, the former is taken as standards for determining transient characteristics of the detected transformer when the detected transformer in real operation is in fault, and transient characteristic precision of the detected transformer can be really reflected. Through mounting the detected transformer at an operation field, the standard source data is sent by utilizing the wireless transceiving system technology to a transient signal processor, and online transient test on the detected electronic transformer is accomplished.