<p>In oil and gas production operations, complex and variable cyclic external loads can trigger bolt loosening, leading to flange leakage, blowout, and other severe accidents. In-service flange leak monitoring is challenging owing to the limited working conditions and equipment integration costs. In addition, most current seal evaluation methods are not applicable to verification in vibrational working conditions. This study proposes a method for predicting the leakage rate of bolted flange joints based on the degree of bolt loosening under dynamic load conditions, which can directly monitor and evaluate the sealing status of flanges in service in real time. This method employs a low-cost, highly stress-sensitive, and stable magnetic measurement method to determine the change in the loosening index of the bolt, analyse its influence on the contact stress of the gasket by finite-element simulation, and propose a leakage prediction model based on critical contact mechanical parameters by simulating contact with a rough surface. The method was verified by a bolt loosening and flange leakage monitoring test under axial cyclic loadings, and provides a new method for evaluating the sealing performance of in-service flanges.</p>