The linear stability of supersoinc mixing layer with the mean velocity profile approximated by the hyperbolic tangent is studied. The convective Mach number of mixing layer is 2 and the temperature ratio is 1. A new classification of disturbance mode of mixing layer is presented to unify the existing results in literature and the new results in this paper. The mixing layer has supersonic and subsonic modes according to the disturbance phase speed. Then the supersonic mode can be classified into mixing mode and acoustic mode. The mixing mode can be further classified into subsonic-supersonic mode and supersonic-subsonic mode according to the relative speed between disturbance phase speed and the free flow speed of mixing layer, which correspond to fast mode and slow mode, respectively. The phase speed of acoustic mode is supersonic with respect to the free flow in both sides of mixing layer and some unstable acoustic modes are found. The acoustic mode has radiation characteristics in both sides of mixing layer and results in a series of expansion and compression fans.