To maximize utilization of ultra high pressure jet energy,ultra high pressure jet flow field is studied with numerical simulation 0n different nozzle combinations.According to bottom flow field velocity vector and bottom hole pressure distribution,flow structure and drilling of nozzle combinations are analysed.A combination of three vertical edge nozzles has high efficiency on rock but is ineffective in cleaning the bottom and it results in wall erosion severely.Center plus a nozzle can clean the bottom and solve the problem of anti.wall erosion,but it requires greater high-pressure fluid displacement.Two vertical side nozzles and a center tilt nozzle or a vertical edge and a central nozzle combination is more favorable.A reasonable nozzle combination is essential in improving drilling efficiency.