The pipeline is widely used in engineering, and the flange bolt is the main way of pipe connection. However, the leakage caused by the loose bolt directly affects the reliability of pipeline service. Monitoring the looseness of the bolt is an effective method to improve the reliability of the flange bolt connection. To this end, the root mean square deviation of the ultrasonic guided wave signal transmitted through the connecting bolt is used as the tightening index for monitoring the bolt loosening in this paper, and the numerical simulation and experiment verification are carried out. The results show that the amplitude of the response signal of the piezoelectric patch decreases with the decrease of the overall preload force. When one or two bolts are loosed, the response signal received by the piezoelectric patch closest to the loosed bolt is the most sensitive, and the corresponding root mean square deviation value is the largest. At the same time, the root mean square deviation value of the receiving piezoelectric array and its distribution will change accordingly with the change of the number and position of loosed bolts. The results show that the proposed method can realize the monitoring of bolt loosening and can preliminarily determine the location of the bolt loosening.