Abstract: In order to meet the requirements of installation and manufacturing, there is a certain gap between the anti-vibration bar and the tube in the part of the U tube of the steam generator. This gap leads to the uncertainty of the restraining strength of the anti-vibration bar, so it is necessary to carry out the sensitivity analysis of the gap. Through the modal analysis in numerical simulation, the spring stiffness is used to characterize the effect of the anti-vibration bar gap on the restraining strength. The results show that when the anti-vibration bar is completely failed, it has great effect on the first natural frequency , which can reduce it by 88.15%. The single point constraint failure has little effect on the natural frequency, and it can only reduce the first natural frequency by 2.65% to the maximum. In each supporting position, the continuous two point constraint near the straight tube has the greatest effect, and the first natural frequency can be reduced by 23.58%.