The influences of the fast gain saturation and the length of SOA on the performances of terahertz optical asymmetric demultiplexer (TOAD) are investigated in this paper. Two operation modes, small width switching window mode and large width switching window mode, are all analyzed. It is found that the fast gain saturation has great influences on both the shape of switching window and the switching energy, and the dependence of the switching energy on the control pulse width are completely different for the two operation modes. In addition, we find that effect of SOA length can greatly reduce the amplitude of the secondary switching window, which indicates that by properly choosing the length of SOA, the switching window width can be reduced and the speed of TOAD can be increased.