When the draghead excavates dense silt，the mechanism of using conventional high-pressure flushing to assist soil breaking is not clear. To understand the actual effect of high-pressure flushing on crushing dense silt，the conventional high-pressure flushing soil breaking test and theoretical analysis are carried out on the dense silt prepared in the laboratory by means of field test and simulation analysis. The results show that the depth and width of soil breaking by high-pressure flushing are greatly affected by the moving speed of nozzle，target distance and diameter，and show different laws.When excavating dense silt，although the auxiliary breaking effect of high-pressure flushing is not obvious，it can reduce the excavation resistance of rake head significantly，high-pressure flushing should be used reasonably according to the actual situation of the site. The results of the test can provide guidance for the design of draghead and the construction technology of dense silt.