Abstract: To study the transient response of electrostatic discharge(ESD) protection device to the EMP with fast rise time, human-metal model (HMM) and transmission line pulse (TLP) current waveforms are widely used for ESD protection devices testing. In this paper, two testing systems were developed. One is an HMM tester, basing on IEC61000-4-2. The other can generate TLP waveform by transmission line charging and discharging. A type of ESD protection device was tested. Transient response waveforms, clamping voltage, clamping time, overshoot voltage were obtained, and the influence of transmission line length on the test results was studied and analyzed theoretically. The experiment results show that: The length of TL has a great impact on the performance testing of an ESD protection device, improper selection can lead to a wrong conclusion; When the ESD protection device is under test, TLP testing is preferred; The length of TL should be less than 8 meters when the ESD pulse testing is used.