Asymmetric shortcomings and production principles of structure switch window of terahertz optical asymmetric demultiplexer (TOAD) with unidirectional control pulse are researched in detail. The shortcomings are well resolved by the TOAD structure of bidirectional control pulse on the basis of theoretical analysis and optisystem simulation. The simulation results show that the switch windows with symmetric waveform and higher contrast are got by bidirectional control pulse TOAD. And double peak phenomena in simulation process are explained.