Ultrafast ga in compression has to be considered besides the linear gain saturation when opti cal pulses shorter than 10 ps are involved in semiconductor optical amplifiers ( SOA).In this paper,the influence of the ultrafast gain compression on the switch ing performance of a THz optical asymmetric demultiplexer (TOAD) due to ca rrier heating and other nonlinear processes in the SOA is numerically analyzed.I t is shown that the breakup effect of the switching window can be circumvented a nd a window width of 2.0 ps with an extinction ratio of 13.7 dB can be achieved at a control pulsewidth of 2.5 ps through careful parameter optimization.