The transient S-parameters of a capacitive shunt and electrostatically actuated millimetre-wave switch were researched.The transient change of these parameters during switching in the gap between switch beam and dielectric layer was derived by a one-dimension mechanical dynamic model from a published paper. Then, the transient change in the gap was used for simulating the transient S-parameters during switching in HFSS software. Finally,this method was applied to a specific switch. Obtained results show that the inserting loss S21 decreases slowly until the end of the pull-in process (from steady value -0.20 dB in Up-state to -1.02 dB over 9.11 μs, 97% of the pull-in time), and it increases quickly to nearly the ultimate value at the beginning of the release process (from steady value -20.1 dB in down-state to -1.16 dB over 1.09 μs, 5.5% of the release time).The transition time from a passing RF signal to a blocking RF signal is about the time of pull-in, and the transition time from a blocking RF signal to a passing RF signal is one order of magnitude less than the releasing time.