Abstract: To address the issues that pitch-catch ultrasonic testing is infeasible for in-service pipe inspection and that pulse-echo testing cannot effectively measure second harmonics, this paper proposes a nonlinear ultrasonic V-scan detection method. By optimizing the incident and reception angles, the nonlinear effects introduced by the measurement system and wedge are suppressed, enabling quantitative characterization of thermal damage in pipes. An incident angle between the first and second critical angles was adopted. Through theoretical analysis of second harmonic generation and propagation, the incident and reception angles were determined to be 55° and 25.4°, respectively. Simulations of thermal damage detection in P91 steel pipe showed that nonlinear parameters increase significantly with rising damage coefficients, whereas fundamental wave amplitudes remain nearly unchanged. Experimental measurements on 10-year-in-service coking furnace thick-walled pipes revealed that the heating side exhibits higher nonlinear parameters than the top and bottom surfaces, and that these parameters increase from the fluid inlet toward the outlet. The proposed method effectively suppresses system-induced nonlinearity, is sensitive to thermal damage in thick-walled pipes, and provides reliable technical support for the safe operation of petroleum refining equipment.