Abstract: When inspecting welded joints of P91 steel pipelines in power plant boilers, the surface temperature ranges from 50 ℃ to 300 ℃. This temperature is higher than 0-50 ℃ that is specified in the current standard of NB/T 47013.3-2023, making it a major challenge for periodic inspection of embedded defects in boilers. This paper fabricated an equivalent calibration test block using P91 steel and designed a special detection probe for high-temperature measurement. The probe wedge was made of polyimide. Then, a system was established for determinating the ultrasonic shear wave velocity of P91 steel within the temperature range of 50 to 300 ℃combined with 900 ℃ high-temperature coupling agent. This solved the problem of large positioning error in high-temperature ultrasonic testing of P91 steel welding joints. The results showed that in the temperature range of 50-300 ℃, the ultrasonic shear wave velocity of P91 steel decreased gradually as temperature rised. At the same temperature, the ultrasonic shear wave velocity of P91 steel was higher than that of 20# steel. Errors generated during scanning baseline calibration and ultrasonic probe fabrication were non-negligible. Therefore, it is necessary to use P91 steel to fabricate an equivalent calibration test block to calibrate the scanning baseline and to manufacture the ultrasonic probe.