Abstract: The activities of solitary IWs were observed in SW'06 experiment. The sensitivities of the normal modes arrival time to solitary internal waves (IW) are analyzed by using the SW'06 environments. Simulation results show that the arrival time of mode 1 is relatively stable. But, there are some higher-order normal modes which arrive earlier than mode 1, and fluctuate with the appearance of solitary IWs. The physical explanation of the phenomenon is given based on ray theory. It is shown that, when thermocline falls down to some depths, those higher-order modes with a group of definite grazing angles mainly propagate above the thermocline and arrive earlier. This phenomenon might be used to monitor IW solitons.