In order to discuss the effect of combustion heat of the products on acoustic radiation characteristics, the underwater combustion for different thermite contents of pyrotechnic compositions were studied with underwater acoustics system. Results show that acoustic energy can be increased significantly by adding the proper content of the thermite, and the optimal thermite content is 25%. Fifteen gram pyrotechnic composition with thermite content of 25% can produce the sound pressure level as high as 158.8 dB, and the acoustic frequency is mainly 0~5 kHz,then the acoustic energy decreases with the increasing of thermite content.