A trial system for follower thrust acted on slender body test piece was designed and applied to solid rocket motor trial. The relational expression between coupled mode frequencies difference and thrust was theoretically derived. It was shown that theo retical results agree with test data， and critical thrust can be predicted based on vibration data from a few of safe and controllable subcritical trials； stiffness coupling caused by transverse component of follower thrust is verified by subcritical trials； thrust should be considered as follower load for the slender vehicle with obvious bending vibration.