Abstract: Slide or rotary steering drilling assembly and deviation dropping drilling assembly with bending angle or bias axis have been extensively applied in horizontal wells,high-deviated wells,extended reach wells,multi-lateral wells and vertical wells,but the application result is not stable because of the insufficient study of the mechanical characteristics and trajectory control theory for this sort of BHA.In order to resolve the trajectory control problems of this sort of BHA,it is necessary to resolve the steadiness diagnosis,movement state diagnosis and parametric analysis of the BHA,the mathematical model of the dynamic analysis,the dynamic model between bit and formation and trajectory forecasting method.This paper discusses the property of the problems,possibly encountered problems and the resolving method.