Abstract: Based on the ultrasonic propagation process, this paper proposes a simplified analysis model of wave line-wave surface dynamic evolution in order to explain the formation mechanism and characteristics of the delayed wave echoes. The source of the moving head wave is the grazing incidence longitudinal wave, and a delayed wave echoes is generated from the head wave (transverse wave) at a critical angle due to the mode conversion (a head wave is transformed to a grazing longitudinal wave) at the lateral opposite surface of the specimen. The main beam energy is gradually transformed into the energy accumulation of the head wave, which results in the delayed wave echoes amplitude being larger than the bottom reflection amplitude. The ultrasonic nondestructive testing gets help with the head wave characteristics, the simplified analysis model of wave line-wave surface dynamic evolution contributes to the analysis of the general ultrasonic testing problems.