Abstract: Bolt is one of common mechanical connection parts. Once a bolt fails unexpectedly, it may cause equipment damage, structural collapse, or even endanger personnel safety. Therefore, the in-service monitoring of bolts has been widely concerned. In this paper, the phased array ultrasonic total focusing technology was applied to the in-service monitoring of bolts, and a special system and array probe were developed to realize the three-dimensional imaging visualization monitoring and change the current status of limited information content of the monitoring data. Aiming at the key issues of massive 3D image data processing, an intelligent analysis method was proposed, and the applicability of decision tree, support vector machine and neural network algorithms were compared and analyzed, an intelligent analysis and evaluation software module based on decision tree algorithm was finally developed and good detection results were obtained.