<p>Wire arc additive manufacturing (WAAM) is widely used for fabricating large-scale metal structural components due to its high material utilization and short production cycle, yet its layer-by-layer deposition process easily induces defects such as cracks and circular holes. The depth of these defects directly determines the load-bearing capacity and service safety of WAAM components. This study aims to realize quantitative evaluation of defect depth in WAAM-fabricated low-carbon steel using metal magnetic memory (MMM) technology. A total of 16 rectangular WAAM specimens with artificial defects were prepared, covering two orientations (longitudinal and transverse) and four defect depths. Room-temperature incremental uniaxial tensile tests were performed, and a TSC-PC-16 magnetometer was used to measure the tangential and normal components of the residual magnetic field (RMF) at each loading step. The distribution and evolution of RMF signals were analyzed, and characteristic parameters were defined to establish the correlations between these parameters and defect depth. Results indicate that RMF signals exhibit obvious anomalies at defect locations. All characteristic parameters exhibit a significant positive linear correlation with defect depth. Longitudinal specimens exhibit stronger magnetic signal responses than transverse ones, which is attributed to higher stress concentration. Incremental loading amplifies the distortion of RMF signals by enhancing the coupling effect between external forces and defects. This study verifies the feasibility of MMM technology for quantitative evaluation of defect depth in WAAM components. It provides a non-contact and high-sensitivity non-destructive testing (NDT) method for the quality control of WAAM-fabricated structures, which is of great significance for safeguarding the safety of critical components in the aerospace, marine, and energy sectors.</p>