Abstract: A new Class Ⅲ flextensional transducer with low frequency,broadband and high power using multimode coupling technique is proposed.By adding an elastic auxiliary flexural beam connected to a concave flextensional shell,and using a bender disk as the end cap,the more useful working modes could be used.The Finite Element Method(FEM) is used to design and analyze the transducer.The FEM results show that the transducer has four main modes in the low frequency band.A prototype of the flextensional transducer is developed,and measurement results show that the Transmitting Voltage Response(TVR) of the transducer is greater than 135 dB in the range of 1.5~5.5 kHz.The maximum transmitting voltage response is greater than 142 dB and the fluctuation of the TVR is less than 6 dB from1.5 kHz to 4 kHz.The result proves that the Class Ⅲ flextensional transducer with decoupling end caps could not only transmit high power in small size,but also achieve low frequency and broadband performance.