<p>Because of the coupling error caused by the adhesive layer between the sensor and the bolt, the ZnO piezoelectric film sensor is directly deposited on the bolt surface to avoid the interference of the adhesive layer. The piezoelectric constitutive equation and elastic theory are used to model the coupling mode, and two methods are used to verify the model. The experimental results show that with the increase of torque, the stiffness of the joint surface of the piezoelectric film sensor increases, and the peak frequency of the admittance signal of the smart bolt increases linearly, which is consistent with the results of the theoretical model; The stability of the peak frequency shift method of admittance signal is further verified by several groups of repeated experiments. Finally, by comparing with the RMSD tightness factor method, it is verified that the admittance frequency offset method has higher accuracy.</p>