Abstract: Along with the progress of drilling technology,bottom hole assembly developed rapidly to control well trajectory. Rotary steering bottom hole assembly is the most advanced.Bottom hole assembly studied in this paper is set rotating steering stabilizer in a common rotate bottom hole assembly to control well path. To develop and use this bottom assembly, this paper provides mechanical analysis of rotary steering BHA, the mathematical model includes forces analysis of rotary steering BHA, BUR calculating, configuration of steering stabilizer and controlling positions of steering stabilizer. This paper provides theory basis for the design of rotary steering stabilizer.