Plane strain finite element method was employed to simulate the generation of acoustic surface wave by the line focused pulsed laser irradiation on the aluminates plate. The temporal displacemental signals on the plate surface were calculated and compared with the waveforms obtained in the numerical model with a surface micro cracks introduced in the near field of laser source. The numerical results demonstrate that the monopolar displacement signals of surface skimming longitudinal wave (sP) and Rayleigh wave (R) become the obvious bipolar signals, as there exists a surface micro defect in the immediate vicinity of laser line source. In addition, the peak-to-peak value of bipolar Rayleigh wave signal is enhanced about two times compared with the initial monopolar one. This dramatic transition can be the basis for the scanning laser line source inspection technique for shallow cracks.