Abstract: Steel corrosion is a primary factor leading to the degradation of structural load-bearing capacity; however, high-sensitivity monitoring methods for the full process of localized corrosion in reinforcing steel remain limited. In this study, an ultrasonic guided wave monitoring method integrating direct waves and coda waves is proposed. Local electrochemical accelerated corrosion tests were conducted on four plain round steel bar specimens, and the characteristics of direct wave and coda wave signals were investigated within a corrosion rate range of 0–20%. A damage index based on normalized wavelet packet energy (Damage Index based on Normalized Wavelet Packet Energy, DI-NWPE) was constructed. Based on the differences in corrosion sensitivity between the two wave components, a weighting strategy was employed to fuse them, forming a comprehensive monitoring index for the entire corrosion process.The experimental results indicate that the direct wave is more sensitive to corrosion in the later stages, whereas the coda wave exhibits better identification capability in the early stage. The transition point of dominant contribution between the direct wave and coda wave occurs at approximately 8%, and the monitoring results of the four specimens show good consistency. Moreover, the fused DI-NWPE exhibits a strong linear relationship throughout the corrosion process, and its fitting performance is superior to that of the indices before fusion.