As one of the most advanced international drilling technologies at present, rotary steering has been extensively applied to large-displacement, long-distance horizontal, and complex structural wells with numerous advantages, such as strong steering deflecting capacity and good borehole quality. Push-the-bit rotary steering is nowadays commercially applied on a large scale. In the bottom hole assembly (BHA) design of push-the-bot rotary steering drilling, flexible pup joints are the main components that exert an important effect on the exertion of steering deflecting capacity of push-the-bit rotary steering tools. The effect of flexible pup joints on steering capacity of BHA used in the push-the-bit rotary steering drilling process was investigated through a finite element analysis and their action laws were summarized. The results indicated that in order to optimize the deflecting capacity of rotary steering drilling tools, flexible pup joints should be installed cooperatively so as to obtain greater steering drilling capacity. In the rotary steering drilling process, as the outer diameter of flexible pup joints increased, the maximum stress of the BHA decreased. Side cutting force of the drill would increase with the length of pup joints, but this would result in degradation of anti-torsion performance of the BHA.