Electrostatic discharge (ESD) is a major failure mechanism for electronic equipment, and transient voltage suppressor (TVS) is the chief means by which the susceptibility to ESD of equipment can decrease. To study the suppressing behavior of TVS to ESD, a testing system was developed, consisting of the ESD generator, the coaxial-test clamp, the 30dB pulse attenuators, current/voltage sensor,and the oscillograph, etc. Using the testing system, the voltage-current behavior of a TVS as a protection device against ESD is described. It is shown that the pulse clamping voltage and the current peak of the TVS increase with the growth of the injected test voltage, and they are linear with the testing voltage. Through comparing and analyzing, it could be concluded that, for the ESD, using the clamping voltage decided by the same response time to estimate the protection performance of TVS is reasonable.