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Continuously welded rail (CWR) has been widely applied in modern railway structures, and its performance is directly related to the safety of railway transportation. After the removal of the rail joints, the CWR cannot expand and contract freely. The longitudinal temperature stress within the CWR turns out to be the core structural parameter in the maintenance and repair of the track. In this study, an ultrasonic theoretical model of the rail temperature force was established, and the mechanism of the critical refracted longitudinal wave to detect the rail temperature stress was analyzed. A numerical simulation calculation was conducted with the three-dimensional finite element model, and the longitudinal wave incidence angle, frequency, and transducer spacing were calculated. The impact of critical factors on the accuracy of refracted longitudinal wave detection temperature stress was investigated. The ultrasonic acoustic propagation under the corresponding stress value of the rail was calibrated to eliminate the influence of residual stress and ensure the accuracy of ultrasonic detection of temperature stress. Finally, an online rail temperature stress monitoring system was built, and a long-term operation was carried out in the Changzhou section of the Shanghai-Nanjing high-speed rail. The monitoring data of the line in 2019 was obtained, and the temperature stress change of the line was recorded. The data showed that the ultrasonic sound obtained by real-time monitoring could accurately reflect the temperature and force changes of the rail. This proposed method is expected to provide a solution for online and remote acquisition of the temperature stress of the CWR. The accumulated long-term monitoring data is of great significance to the prevention plan for rail expansion and rail breakage.