The excitation of terahertz (THz) surface mode at oblique incidence of two-frequency laser radiation on semi-bounded plasma is studied. It is shown that if frequency difference of laser fields coincides with the surface eigenmode frequency, the significant increase surface wave (SW) field occurs. It is found that additional amplification of SW field occurs when the s- and p-polarized laser radiation is incident at the angle of total reflection. The SW energy flux density increases most significantly for p-polarized laser radiation when it is incident at the angle of total reflection on the near-critical plasma with rare electron collisions. It is shown that in this case the SW energy flux density can significantly exceed the laser intensity.