Abstract: In slim-hole drilling, such downhole troubles as pipe blocking and sticking frequently occur, due to the small clearance between the bore wall and drilling pipe. A 165.1mm dimension-variable hydraulic stabilizer is developed to minimize such problems, and its working mechanism and features are illustrated, its mechanism of preventing pipe blocking and sticking are also analyzed. The hydraulic stabilizer has strong wear resistance and long using life, and the BHA can be adjusted according to the real drilling conditions, therefore the changes of BHA can be minimized as well.