Abstract: Aiming at the problems of high environmental resistance and low efficiency in the process of underwater rock breaking.a new method of rock breaking by cross medium jet.which called gas-assisted water jet.is proposed. The basic characteristics of underwater gas-assisted water jet are analyzed.and the effects of working parameters and nozzle structure parameters on rock breaking performance and flow pattern of underwater gas-assisted water jet are explored based on relevant test bench. The results show that the rock fragmentation is caused by the combined action of water hammer pressure in water phase and micro jet in gas phase. When the airflow pressure is less than 0.4 MPa.the rock fragmentation caused by the cavitation collapse microjet is more sensitive to the change of airflow pressure. However.when the airflow pressure is greater than 0.4 MPa.the rock fragmentation caused by the jet impact dynamic pressure is more sensitive to the change of the airflow pressure. The change in the structure of the constricted section of the outer nozzle has less influence on the development of cavitation than it has on the jet impact dynamic pressure. Increasing the exit diameter of the outer nozzle.the clustering of the jet first increases and then decreases. As the length of the straight section of the outer nozzle increases.the degree of cavitation in the underwater gas-assisted water jet gradually increases.but at the same time.the energy loss caused by the gas and liquid momentum exchange and frictional resistance increases. Compared with the common submerged jet.the maximum rock breaking volume increased by 150% and the specific energy consumption decreased by 40%. With the increase of airflow pressure.the volume of rock fragmentation first increases and then decreases.and at the same time.the specific energy consumption first decreases and then increases. The best value is 0.4 MPa. Extend the erosion time.the rock fragmentation volume will increase and gradually slow down. The erosion time is set to 30 s.which can take into account the rock breaking performance and economy of the underwater gas-assisted water jet. The optimum structure parameters of the outer nozzle are:exponential contraction section.3.5 mm outlet diameter and 10 mm straight section length.