Localized corrosion leads to a local reduction in wall thickness in pipes, compromising their load-bearing capacity. In this study, a quantitative detection method for full-circumferential localized corrosion in pipes based on longitudinal guided waves is proposed. Based on theoretical analysis of the interaction between full-circumferential localized corrosion defects and longitudinal guided waves, the proposed method consists of two procedures. First, the theoretical relationship between the time-of-flight (TOF) difference of L(0,2) modes reflected from the front and back edges of the corrosion defect and the axial length of the corrosion defect was established using the pulse-echo method, enabling quantitative evaluation of the axial length. Subsequently, with the axial length determined, the theoretical relationship between the TOF variation of the L(0,1) mode and the depth of the corrosion defect was established using the pitch-catch method, allowing quantitative estimation of the depth. The effectiveness of this method was verified experimentally. The results demonstrate that the measured axial lengths and depths agree well with the preset values, with maximum relative errors of −1.27% and 9.52%, respectively. The proposed method overcomes the limitation of conventional guided wave methods, which are generally difficult to accurately quantify the axial length and depth of full-circumferential localized corrosion defects simultaneously.