Abstract: The stress of the steel plate was detected by measuring the coercivity of the steel plate, the stress detection mechanism based on coercivity was analyzed, and the coercivity detection principle was studied. The coercivity of the specimens of Q235, X70 and X80 steel plates under different stresses was detected by the coercivity detection system in the range of elastic deformation, and the data were curve-fitted by the least squares method. The results show that the coercivity of the steel plate changes linearly with its stress which is consistent with the theoretical analysis of the results.