【Objective】Ultrasonic testing serves as a primary technique for detecting rail defects, though the complex interference effects within the near-field zone reduce detection accuracy.【Method】The analysis constructs a theoretical sound field model for ultrasonic propagation inside the rail using Huygens" integral principle and performs numerical calculations under various frequencies and chip sizes. Tests verify the responses of defects at different depths in the rail head.【Result】Increasing the rectangular chip size from 10 ×10 mm to 20 ×20 mm extends the near-field zone length from 11.9 mm to 51.4 mm. Raising the probe frequency from 1 MHz to 4 MHz more than doubles the near-field zone length. Within the near-field region, defect echoes blend with the initial wave and become difficult to distinguish, whereas in the far-field zone, defect signals separate distinctly, thus improving detection accuracy.【Conclusion】Probe frequency and chip size exert a quantitative influence on the boundary between near-field and far-field zones. Shallow defects show greater susceptibility to near-field interference, while deeper defects achieve clearer signal identification in the far-field region..