The blades on the plane are one of the most important parts of the engine, in the course of service, due to high temperature, strong vibration and great centrifugal force and so on. The using environment is very bad, so it is easy to produce fatigue cracks in the welding site and the near surface of the root, which will seriously affect the blade of the work intensity and fatigue life, and even the safety of aircraft structure, causing a huge security risk. Therefore, it must be tested. In order to solve the problem of the rapid detection of aircraft engine in situ cracks, and gett the relationship between feature information and detect depth, the laboratory experimental platform was built, laser was used to excite laser ultrasonic signals on a range of aviation aluminum plates with different depth defects, the collected signal was processed by wavelet de-noising, and the band energy distribution of the reflected echo signal was studied by using wavelet packet. The results show that the energy of reflected echo signal is mainly concentrated in the S80～S87 band. When the depth of defect is 0.2 mm to 0.4 mm, the energy is mainly con-centrated in the adjacent bands. When the depth of defect is 0.5 mm to 0.7 mm, the energy is mainly concentrated in the two bands. This method provides a way to quantify surface micro-defects by ultrasonic signals, which will lay a foundation for the future analysis of crack depth from band energy.In order to avoid the interference of other irregular cracks, the cracks of the aviation aluminum parts are used as artificial way for producing. The overall size of the specimen is 200 mm×80 mm×100 mm, the width of the defect is 0.15 mm, the range of the defect depth is 0.2 mm~0.7 mm, step size is 0.1 mm, and the total number of the specimen is six. After the experimental data is proposed, choosing the reflected echo signal for analysis, perform-ing wavelet packet transform, the decomposition layer is 8. The percentage in the S80-S87 band is 89.77%、91.82%、91.41%、90.94%、90.19%、and 87.86%. The result shows that most of the energy is concentrated in the first eight bands. Therefore, the paper selects the first eight bands for analysis.In order to analyze the distribution characteristics of the different depth defect and the band energy, the energy distribution of the first four bands of the defect depth of 0.2 mm to 0.4 mm is plotted in Fig, according to the spec-trum, getting the center frequency were 3.14 MHz, 2.58 MHz, 2.17 MHz. These frequencies are located in the S83, S82, S82 band, respectively, which are the largest energy band, but the energy distribution in the adjacent segment S82 also accounts for a larger proportion. When the depth of the defect increases from 0.2 mm to 0.4 mm, the center frequency decreases gradually, and the sum of the energy of the center frequency band and the adjacent higher en-ergy band increases gradually.