Laser butt welding of 7075-T6 aluminum alloy is carried out by Nd:YAG laser, and the welding sample quality is measured by ultrasonic fault detector. The ultrasonic echo data are gathered and analyzed by experiential waveform analysis, and different typical echo data are collected at different processing paramenters of laser butt welding. The relation between echo feature and processing parameters of laser butt welding is also obtained. The research results show that processing parameters of laser butt welding including laser voltage, laser defocusing amount and welding speed are very important to the welding quality. The optimum processing parameters can be obtained by analyzing the ultrasonic echo data. The research provides a new method to measure the quality of welding seam.