Abstract: The article briefs the machanism on whici trenching can be carried out through underwater explosion by way of piling steel pipes. The general law of submerged explosion is expounded, backed up by explosion practions and formular derivation concerned.It gives the formulars for calculating the amount of explosive to be used, assuming that fluid is uncompressible. With the development of construction tecnology, underwater explosion trenching method will be even safer and more re1iable.