The interface and confined phonon modes of the wurtzite quantum cascade laser are investigated using the transfer-matrix method.One group of interface modes and two groups of confined modes are presented in the longitudinal optical frequency domain,and the interface modes can be transited into the confined modes for two lowest frequency modes.The dispersions and electronic potentials of the interface and confined modes are obviously different,and the scattering rate induced by the confined mode is comparable to that induced by the interface mode.