Abstract: An underwater detector was designed based on the technology of laser-induced sound by underwater explosion. The mechanism of underwater explosion sound by laser was studied, and then the detecting experimental system was built. The experimental results show that the laser-induced sound signal might be produced by high-power pulse laser breakdown under water, and that the spherical laser-induced sound signal can be changed into the plane wave signal by the acoustic reflector inside the detector. The laser-induced sound signals from the detector are found to have the characteristics of narrow-beam direction, high range resolution and long detection range by numerical calculation, which meet the requirement of underwater detection.