Base on the principles of phase array of piezoelectric sensors, experimentation of lamb wave phased array in large thin aluminium plate was processed by an SO mode Lamb wave resulted from high frequency narrow bandwidth signal. Different locations on large thin-wall structural were effectively focused through adjusting the signal transmission delay phase of elements. The concept of dead zone of phase array is further clarified and several measures to reduce the dead zone detection were given, which was favorable to improve signal-to-noise ratio of Lamb wave detection.