[12] analyzed the guided wave propagation in a multi-layered structure. [PERSON] et al. [13] applied UGWT to monitor the structural health of aircraft composite structures. [PERSON] [14] conducted an in-depth study of ultrasonic-guided wave mechanics in composite material. Ultrasonic-guided wave testing can be directly applied to oil and gas pipeline corrosion detection at room temperature. Studies of nonlinear guided waves used to detect microcracks in structures and to characterize materials have been reported [15; 16]. However, because of the poor high-temperature resistance of ordinary piezoelectric ultrasonic probes, which can only operate normally below half of their Curie temperature, they cannot be directly in contact with high-temperature pipelines [17]. To solve this problem, several scholars [17; 18] devoted themselves to studying high-temperature piezoelectric materials, such as bismuth titanate and lithium niobate materials, which can reach a Curie temperature of more than 500 deg C. The weakness of the present piezoelectric ultrasonic probe-based UGWT is that it cannot be directly applied to the thickness measurement of high-temperature pipelines. Therefore, scholars have turned their attention to making the temperature at the site of contact with the ultrasound probe cooler. [PERSON] was the first to propose the use of waveguide rods as a temperature buffer structure for high-temperature specimens, applied for a patent, and then used incomplete welding. He used a cylindrical waveguide rod welded to the surface of the workpiece. [PERSON] [19] proposed a non-contact method to measure the wall thickness of hot steel pipes. [PERSON] et al. [20] also used a waveguide rod of the same material as the pipe under test as a temperature buffer structure. She welded it vertically to the outer surface of the pipe under test in a fully welded manner. [PERSON] [21] designed a rectangular waveguide rod to isolate the sensor and piezoelectric device from the high-temperature measurement area without the need for additional cooling equipment. [PERSON] et al. [22] and [PERSON] et al. [23] both proposed a method to excite shear horizontal (SH) waves. The waveguide rod is not only a structure for buffering high temperature but also a medium for guiding the wave transmission. In terms of buffering temperature, [PERSON] [24] argued that the buffer structure should have a high melting point because it is in direct contact with the high-temperature pipe. Moreover, the heat transfer coefficient of the buffer structure is as small as possible to achieve an excellent cooling effect. In terms of the perspective of wave propagation, [PERSON] et al. [25] believed that the buffer structure should be flexible. The waveguide rod itself does not generate vibrations to avoid traveling excess ultrasound signals. In addition to that, the waveguide rod can transmit the stress wave, and the waveform distortion is as tiny as possible so that the signal can be read easily. In terms of numerical simulation, some scholars were dedicated to studying the propagation process of ultrasonic-guided waves in plates and pipes [26; 27]. [PERSON] et al. [26] conducted numerical simulations of the propagation characteristics of AE signals in waveguide rods using ANSYS and found that AE signals are affected by the shape and size of waveguide rods to varying degrees. The finite element method, as a numerical analysis method with a theoretical basis and widely used effects, can be used to solve problems in the field of acoustics that cannot be solved by analytical methods [28]. In this paper, a rectangular waveguide structure for buffering high temperatures was designed. Optimum material and size were determined to ensure the ultrasonic testing at a high temperature of 500 deg C. Ultrasonic propagation through the designed waveguide rod and specimen was calculated with COMSOL Multiphysics. Wave propagation, reflection, attenuation, and dissipation in the combined structure were displayed.