The electromagnetic acoustic resonance (EMAR) method provides accurate and stable evaluation in high temperature environment, and it is an effective tool for online monitoring. In this study, the EMAR method and the superposition of the nth compression (SNC) for data processing are applied to online monitoring of pipe wall thinning, and the accuracy and reliability of the measurements are demonstrated through field tests using a large-scale corrosion test loop at high temperature. To measure the thickness of pipes with complicated wall thinning, the SNC extracts thickness information from the spectral responses of the EMAR. Results from monitoring test show that EMAR with SNC can evaluate pipe wall thinning with an accuracy of 10 μm at 165°C. In addition, time evaluation of evaluated thickness decreases monotonically all over the test duration, which indicates high stability of this measurement technique.