The reaming drill bit in slim hole is a key tools of re-drilling in existing well. Hydraulic diameter adjustable drill bit is designed to overcome problems of small underground space and insufficient power source with analyzing the stiffness and strength of drill bit body by numerical simulation method and adopting some new structure: overall knife block, slope structure to adjust the diameter of drill bit and so on. This drill bit can ream the well hole diameter form 118mm to 146mm and it is verified by field test that this drill Bit has a easy operation, works reliably and reams even well hole diameter .It is a new species of reaming drill bit.