The influence of surface barrier on the third harmonics ac susceptibility ͑ 3 ͒ is studied numerically. The surface barrier is described by a critical current density in surfaces which are higher than the inside one. The model can act as the critical state one, the flux creep one, or the flux flow one when temperature ͑or field͒ changes. The numerical results based on the model are more close to the popular experimental data probing the harmonics as a function of temperature ͑or field͒. Besides, the surface barrier will lead to new peaks in the real and imaginary parts of the third harmonics, which are the finger signature of the surface barrier. Comparison of the peaks with those of the real part of elemental harmonic shows that they are located at the same temperature where dips in Ј caused by the surface barrier occur, which is in well agreement with experiments.