The conic extruding bit usually be put into use when adopting percussionrotaryextruding drilling method to drill. This bit has very higher drilling efficiency than others in loose soil. But it's drilling efficiency would lower obviously in the middle and stiff plastic soil. To counter this problem, the writers analyzed the factors of soil that would effect drilling efficiency and the effecting reason. The shortcoming of the PRE bit have been find, and the cam PRE bit has been designed. The use range of the PRE drilling method be bind in this paper.