Abstract: the rotary steering drilling system represents the advanced level of the directional drilling, and the development direction of the well-path control technology. In order to solve the problem of the well-path control for a rotary steering drilling, the rotary steering bottom hole assembly (RS-BHA) analysis is made in this paper. Applying the beam-column theory, a three-dimensional mechanical model for a typical RS-BHA is established. The relation between the bit side force and the steering parameter is derived, and based on the limiting curvature method, the building capability of the rotary steering tool can be predicted, and the steering parameter for the planned well-path is calculated. The RS-BHA can be optimized through a mechanical analysis, and the rules of the well-path control are obtained, which can provide a theoretical basis for the design of the rotary steering drilling tool and the automatic control in the directional drilling.