Abstract: Casing drilling technology is essential to borehole which can reduce shrinkage, collapse and save the cost. It is very essential in the soft and heterogeneous layers. A new borehole reamer without lifting casing is introduced in this paper. The drilling tool is composed of drilling pipe joint, sliding block, diamond cutting block and guiding rod. The authors discuss structure and working mechanism of the new reamer. Diameter of the reamer can be changed by sliding core and sliding block. This characteristic is different from conventional reamer. The matching calculating between drilling fluid volume and the weight of diamond cutting block has been done through hydraulics method. This formula can provide mathematics basis for the reamer design. The results of the field application show that this borehole reamer can meet the reaming requirements, release the labor intensity and improve drilling efficiency.