Abstract: In this paper, the surface microcrack detection of aluminum alloy 6061 with different depths was studied by laser ultrasonic detection technology. The defect signals of surface cracks with different depths were obtained by building a laser ultrasonic testing platform. The characteristics of the defect signals in time domain were analyzed. The results show that:the crack depth can be qualitively assessed by using the time difference of reflection echo, so that the quantitative detection of micro-crack depth on metal surface can be realized; by testing the surface defects of steel Q235 at different depths, the correctness of using oscillating time difference to quantitatively detect and characterize the defect depth is proved. The research results of this paper have potential application value.