Abstract: Piezoelectric ultrasonic technology can effectively detect various cracks and crack-like defects in pipelines. It has been widely used in the in-line inspection of oil pipeline defects due to its advantages of high detection sensitivity and accuracy. This paper analyzes and presents the fundamental principles of piezoelectric ultrasonic in-line inspection technology. It provides an overview of the development of this technology by major well-known in-line inspection companies abroad, as well as an introduction to the technical characteristics and performance of their equipment. Additionally, the paper summarizes the current status of piezoelectric ultrasonic in-line inspection technology in China and the future development direction of this technology. It offers insights and recommendations on the advancement of this technology in inspecting long-distance pipelines in China.