Abstract: In nuclear power plants, the loss of pretightening force of high - energy steam pipeline bolts and foundation bolts of major equipment may lead to the inability to ensure the safe and reliable operation of key equipment under special working conditions, especially in extreme accident situations. This paper systematically analyzes the technical principles of the conventional tension method and the ultrasonic method, and proposes a pretightening force detection technology that combines the traditional tension method with the automatic wave - seeking transverse - longitudinal wave fusion. Through the bolt pretightening force calibration test bench, a linear relationship curve between the pretightening force of the VVP lateral restraint foundation bolt (M80) and the ultrasonic time - ratio is established. Experiments show that this technical method has a relative error ≤ 5% under the working conditions of high - energy steam pipelines, and the detection accuracy of the pretightening force is 30% higher than that of the single tension method. At the same time, it can realize zero - tensioner in the detection process of large foundation bolts, effectively avoiding the damage to large bolts caused by multiple stretchings. It provides a new method and new ideas for the detection of pretightening force of large bolts in nuclear power plants.