Abstract: Ultrasonic first-pulse phase reverse is widely used in the field of construction engineering quality inspection due to its accuracy, simplicity and easy operation.Combining theoretical analysis, numerical simulation and experimental verification, the causes of phase reverse is studied. The research results show that the phase reversal phenomenon is caused by the diffracted phase reversal angle of the diffracted longitudinal wave; near the diffracted phase reversal angle, the amplitude of the first-pulse decreases first and then increases. As a result, ultrasonic first-pulse phase reverse has been completely and correctly explained, which provides a theoretical reference and more powerful data support for its application.