The abstract should summarize the contents of the paper in short terms containing 235–450 words. This document explains and demonstrates how to prepare your camera-ready manuscript for IFEDC 2020. The best is to read these instructions and follow the outline of this text. The text area for your manuscript must be 17 cm wide and 25 cm high (6.7 and 9.8 inches, resp.). Do not place any text outside this area. Use good quality, white paper of approximately 21 x 29 cm or 8 x 11 inches (please do not change the document setting from A4 to letter). The flange bolt connection is an important sealing method for wellhead equipment, and its sealing reliability is directly affected by the magnitude of the pre-tension of the connecting bolts. Therefore, the efficient assessment of the stress state of flange bolts under field conditions is a crucial link in evaluating the sealing performance of flanges and predicting the risk of flange failure. This paper first compares the methods for detecting bolt axial stress and then selects a combined magneto-acoustic method to study the relationship between magnetic signals, ultrasonic signals, and bolt axial stress. A quantitative detection system and evaluation method for bolt stress based on the combined magneto-acoustic approach are proposed. Experiments have shown that there is a linear relationship between the magneto-ultrasonic signals and the bolt axial force. For bolts of different models but the same grade, the changes in magnetic signals and the time-of-flight difference of ultrasonic signals are approximately equal. For bolts of the same model but different grades, the changes in magnetic signals and the time-of-flight difference of ultrasonic signals are different. This method can conveniently and accurately measure the stress changes of bolts in the Gas extraction trees under field conditions, providing technical support for further conducting flange sealing performance diagnosis and assessment, and ensuring the safe operation of wellhead equipment in oil and gas wells.