The equipment is consisted of steel casing, under pressure mechanism, rotary drilling rig, clamping device and soil bucket. The steel casing pipe is driven into the soil and the under pressure mechanism and the rotary drilling rig are clamped to the top of the steel casing through a clamping device. The rotary drilling rig and the under-pressure mechanism continuously drive to designed depth. After that, soil bucket is used to take and dispose the soil. With this flexible excavation method, the cost can be greatly reduced.