Abstract: A consummated method was proposed to overcome the difficulties of determination of the amplitude of ultrasonic signals using ray-tracing method. Wave pattern transform when ultrasonic waves are incident on a plane interface was analyzed based on Snell’s law，and the change of wave amplitude after pattern transform was calculated in sequence, then parameters and tracing state of the ray was updated. Ultrasonic testing of a steel plate with straight beam probe using immersion method was calculated and investigated. The results show that the calculated signal characters are consistent with that of experiment in a general way, with a deviation about 5dB on change of amplitude. The method offers a technique foundation to calculate the amplitude of ultrasonic signals using ray-tracing method.