Abstract: LC9 aluminum alloy is the primary structure material in the field of aeronautics and astronautics. The traditional detection methods of this material are all based on damage to the material, and have the disadvantages of long experimental cycle and limit sampling area. Thus they are especially not suitable for on-site inspection. LC9 aluminum alloy after solid solution treatment was chosen as the research object in the experiment. The relationship between the microstructure and ultrasonic velocity of LC9 aluminum alloy under different aging condition was analyzed through testing. The results show that the ultrasonic velocity varied corresponding to different microstructure. The ultrasonic detection could be used to determine the microstructure of other materials.