The relationship among the slow-wave and echo pulse broadening effects in reflected acoustic waves and the width of the interface layer of phononic crystal has been theoretically investigated. It has been observed that not only the slow time for the reflected acoustic wave but also the echo pulse broadening reaches a saturation point as the size of the phononic crystal increases. From the viewpoint of the acoustic wave, there is an interface layer in the crystal that determines the slow-wave and the echo pulse broadening effects. The longest slow time, which is the time needed for transmitting 0.08 periods of the phononic crystal, occurs when the width of the interface layer is 1.89$\lambda $. The width of the echo pulse is broadened no more than 0.13 periods when the interface layer width is about 2.69$\lambda $.