As an important way to solve the problem of abandoning wind power and photovoltaics, PEM electrolytic hydrogen production technology has been widely used and is developing in the direction of large amount of hydrogen production and high hydrogen output pressure, so it also increases the risk of hydrogen pipeline leakage, and it is urgent to match the detection technology. The ultrasonic guided wave in the tubular structure has the advantages of low attenuation and long distance propagation. Compared with the point-by-point scanning of ultrasonic body wave, it can efficiently detect the defects or damage inside the hydrogen transport pipeline. In this study, absorption boundary construction and two-dimensional Fourier transform are used to determine the leakage of hydrogen pipeline by means of mode conversion. The results show that after T (0,1) mode passes through the square hole, F(n,1) mode and F(n,2) mode are formed gradually due to the mode conversion of ultrasonic guided wave. In addition, T (0,1) mode passes through the square hole and reflects, and F(n,1) mode and F(n,3) mode are gradually generated due to mode conversion. This provides a theoretical basis for leakage assessment of hydrogen pipeline based on ultrasonic guided wave mode conversion.