Abstract: This paper focuses on the purpose to find out the influence of ultrasonic head wave amplitude on measured sonic time value. With analog type ultrasonic detector, the sonic time value monitored leans towards higher when the ultrasonic head wave amplitude decreases. Furthermore, its degree of increase is higher while the head wave amplitude becomes narrower. In plane test method, the value likewise leans towards higher when the head ultrasonic wave decreases. However, the average measured speed of sonic wave obtained from the time-distance curve is not relevant to the constant set value of the head wave amplitude, adopting constant amplitude value in the increase of test distance step. With the intelligent ultrasonic detector, when the head wave amplitude changes in certain range, the duplication of sonic time reading is satisfactory, but either oversized amplitude or too undersized amplitude of head wave leads to reading deviation.