Abstract: Pressure-bearing components made of P91 steel are subjected to prolonged exposure to high-temperature service environments, making them prone to creep damage, which poses significant risks to the safe operation of coal-fired power units. Conventional ultrasonic testing exhibits low sensitivity in detecting incipient creep damage due to the micro- or sub-millimeter scale of microstructural variations and voids. Leveraging the ferromagnetic properties of P91 steel and the high sensitivity of magnetic Barkhausen noise (MBN) to microstructural changes, this study proposes a nondestructive testing method based on MBN for assessing incipient creep damage in P91 steel. Creep tests at 600 ℃/110 MPa established the material’s creep life. Specimens subjected to creep exposure under 40% of total life were prepared, with microstructural evolution characterized via metallographic analysis. The results indicate that key figure features (RMS value, peak-to-peak amplitude, envelope peak value, and ringing count) decreased, then increased, and ultimately stabilized with creep time. This behavior conforms to an empirically fitted cubic relationship and directly correlates with P91’s microstructural degradation process. Comprehensice evolution demonstrates that RMS value provides optimal sensitivity, robustness and linearity for assessing P91 steel’s early creep state.