Abstract: In the medium radial horizontal well and large tilt well,the mechanical characters of BHA is under large deformation with small strain.This paper presented the solution of BHA under large deformation by means of beam-column theory,on the basis of solution of BHA under small deformation,through the developing of theory.The formula of three bend moments of BHA under rotary drilling,bit side forces and tilt are given.The general formula of three bend moment fitting to stearable BHA with several structures is developed,and the solution of BHA with single bend sub and tow stabilizers is shown as an example.The analyzing conclusions are given for design and selection of stearable BHA.