Transient error and tailing of an ECT acquisition unit affects the correct operation of relay protection when the fault occurs. This paper simulates and verifies that the zero crossing transient error is the root cause of the tailing phenomenon, and this error is caused by the circuit integral par of the Rogowski coil. In principle this cannot be completely eliminated. Therefore, we propose improving the transient transmission characteristics of this kind of acquisition unit by increasing the attenuation time constant of the hardware integration circuit and enabling the integration attenuation algorithm of the software integration dynamically. In order to test the transient characteristics of the improved acquisition unit, we propose a continuous time-domain differential technology to simulate the output signal of the Rogowski coil, and realize the measurement of transient error by the synchronous technology of the compensating fault phase. When undertaking an engineering application of ECT based on the Rogowski coil or replacing the acquisition unit, it can provide a complete set of test means. This work is supported by the National Natural Science Foundation of China (No. 51477044).