In the experiments of the crack depth testing of concrete by ultrasonic method, the rule of ultrasonic first pulse phase reverse had been discovered when the distances between the two transducers which were placed on the two sides of the crack were changed. Based on the special conditions in which the crack was filled by water during the experiments of the crack depth testing of concrete by ultrasonic method, when the distance between the two transducers is less than twice of crack depth, the phenomenon of ultrasonic first pulse phase reverse could not be found. Thus, new explanation of the mechanism of ultrasonic first pulse phase reverse is proposed. The ultrasonic first pulse phase reverse is due to the fact that the refracting transversal wave arrives to the receiver transducer faster than that of the refracting longitudinal wave.