Abstract: Rotary steering tools are the most advanced steering tools currently. Study on the deflection ability of rotary steering systems (RSS) is useful for the development and application of RSS. The mechanism of bit steering was analyzed under bit lateral force and the angle, based on which, the deflection ability of two kinds of RSS were optimized. The result showed that the backup type RSS should be matched with side aggressive bit to quickly and accurately adjust the trajectory, so it performed well in soft formation with small weight on bit (WOB). Long gauge protection bit was fit for directional RSS so that the WOB could be guided to the direction of bit angle, and the RSS had steady deflection rate and the well trajectory was predictable. The applications of RSS verified the analysis results.