Abstract: The metallic structure is prone to micro-cracks in the early service process. The detection and tracking of the growth direction of early micro-cracks has important practical significance to avoid sudden disasters caused by structural failure. In this paper, based on the non-collinear ultrasonic mixing method, the research on the localization and characterization of micro-crack expansion in metallic materials is carried out. In the study, a non-collinear mixing mode is selected to produce mixed longitudinal waves by interaction of two series of shear waves, and the mixing effect and signal propagation are verified through experimental measurements. The prefabricated vertical and oblique micro-cracks in aluminum alloy 7075-T6 are selected as the research object, and the ultrasonic mixing nonlinear parameters at each measurement point are extracted and normalized to be drawn into a scanning image to characterize the length and expansion direction of the micro-cracks. Finally, the scanning image and the metallographic observation of optical microscope are compared and analyzed. The research shows that the ultrasonic mixing method can effectively locate and track the micro-cracks in different directions in the aluminum alloy material, which can provide an effective method for monitoring microcrack growth in metallic structure.