Abstract: Aiming at the ultrasonic scattering attenuation problem caused by the grain structure of high-temperature alloys, this paper proposes a two-dimensional polycrystalline microstructure modeling method based on the Voronoi algorithm. A finite element model of ultrasonic phased array for GH738 nickel-based high-temperature alloy is constructed by using the COMSOL-MATLAB co-simulation. This model is used to systematically investigate the effects of average grain diameter (100~300 µm), probe frequency (2~10 MHz), number of array elements (6~16), array element spacing (0.25~1 mm), and focal depth (1~4 mm) on ultrasonic wave propagation and attenuation. The results indicate that the attenuation coefficient increases with grain diameter, frequency, and the number of array elements, but decreases with focal depth. The error between the simulation results and the measured data is less than 8%. Furthermore, the ultrasonic intensity at the focal point increases with the number of array elements and decreases with larger grain diameters and wider element spacing. The experimental results confirm the variation trends observed in the simulations, thereby validating the accuracy and reliability of the proposed model. This work provides a theoretical basis and practical guidance for optimizing ultrasonic testing parameters for superalloys.