Casing used in oil and gas wells is usually of large outer diameter, thin pipe wall and high stiffness, which can make its running down operation very difficult and complex with unexpected obstructions. Especially, when the casing annulus clearance is small, large borehole curvature or any irregular wellbore may cause high running resistance, making the casing running operation more complicated. In field practice, a tripping bottom hole assembly(BHA)is often used to correct the wellbore, smooth the borehole wall, and prepare for a smooth running of the casing string. The design and structure of the tripping BHA are important for its function and performance. In this study, a new design and evaluation method of the tripping BHA was proposed. Firstly, a static model of the tripping BHA and casing string was established. Then, according to continuity and boundary conditions, the spatial configuration of the tripping BHA and casing string in actual wellbore was figured out based on a weighted residual method, and the deformation energy of the tripping BHA and casing string with the same length was calculated and compared. Finally, an evaluation method of the tripping BHA based on equivalent stiffness was formulated to realize the design of a new tripping BHA. The results show that a reasonable tripping BHA should have a good operation effect for small hole size, but for a well with large hole size and small casing-wellbore clearance, the tripping BHA may still have a poor operation effect, and therefore it is necessary to use a reamer to enlarging the borehole.