Disclosed is a method of processing an S-parameter to analyze transient phenomena in a passive network, the method including: generating an extrapolation function related to a real part of a measured S-parameter signal; generating an expanded S-parameter signal by the extrapolation function; and setting an optimum degree and an optimum expansion frequency of the expanded S-parameter signal. Thus, the extrapolation function where continuity from the real part of the measured S-parameter signal is ensured is used such that causality in an impulse response of the expanded S-parameter signal can be maintained without problems.