Abstract: To address the condition monitoring problem characterized by weak detectability and poor traceability of early-stage degradation and micro-damage in weak regions of welded pressure vessels during service, a nonlinear ultrasonics (NLU) detection and evaluation strategy based on harmonic features was proposed, in which the equivalent nonlinear parameter β′ = A₂/A₁² of the fundamental amplitude A₁ and the second harmonic amplitude A₂ was adopted as a uniformly calculated and defined state trend characterization metric. An explicit finite element model incorporating mechanisms such as material elastic response nonlinearity and closed microcrack/micro-interface contact acoustic nonlinearity was first established, and the differential effects and coupling characteristics of multi-source mechanisms on harmonic generation and parameter feature responses were systematically analyzed. The investigation shows that contact acoustic nonlinearity exhibits stronger nonlinear effect characteristics; under fixed ultrasonic propagation path and excitation conditions, β′ is appropriately extended and interpreted as an equivalent indicator of multi-source nonlinear coupling intensity to clarify its physical meaning and applicability boundaries. Subsequently, an NLU detection link platform was constructed and calibrated, and repeated coupling detection validations were conducted on gradient fatigue specimens. The results show that β′ exhibits a reproducible “double-peak” evolution characteristic with the fatigue process (the ratio of fatigue life N to the fatigue life at failure N_f, i.e., N₀ = N/N_f), providing higher sensitivity and discriminability to early-stage state variations of structural materials compared to linear acoustic indicators. This study provides an interpretable parametric feature baseline for the engineering application of “periodic inspection and trend interpretation” in welded pressure vessel structures and lays a foundation for subsequent data-driven/intelligent condition assessment.