The three-dimensional time-domain computational approach is employed to calculate the transmission loss of reactive silencers without and with flow. The detailed procedure is explained as follows: a pressure pulse is imposed at the inlet of silencer, and then the three-dimensional unsteady fluid dynamics computation is performed to acquire the pressure perturbations at the upstream and downstream of silencer. Finally，the incident and the transmitted pressure signals in the time-domain are transformed to the frequency domain by means of FFT, therefore the transmission loss of silencer can be determined. The transmission losses of reactive and perforated tube silencers without and with flow are calculated using this approach, and the numerical predictions agree reasonable well with the experimental results in the literature.