Abstract: According to wellbore trajectory control theory, thetype of BHA can be changed through modifying thelocation and OD of stabilizers to realize building up,dropping or holding hole angles. As an importantorientation tool, adjustable stabilizer is especially suitablefor modern drilling. This paper introduces the structureand principles of operation of a hydraulic adjustablestabilizer and analyzes the characteristics of its actuatingmechanism. It is believed that correct allalysis of theforces on its mainshaft(main piston) is critical for design.Taking automatic deviation angle stabilizer as an example,structure of mainshaft and its reset unit are improvedthrough actual calculation.