The composite defect existing together with the stress is one of the important factors which affects the safe operation of pipeline. The dual magnetic field method can be used to judge the degree of stress damage at composite defects. But the stress signal extraction method needs to be solved urgently. In this article, the magnetomechanical relationship in the J-A theory is introduced into the magnetic charge model. The magnetic signals of composite defects under different stress and external magnetic field are analyzed and calculated. The stress signal extraction model of composite defects based on the ratio of dual magnetic field signals is formulated. The ratio factor is proposed to evaluate the stress level at the defect, and the systematic experimental evaluation is carried out. The results show that the strong magnetic signal is not sensitive to the stress at the defect, and the signal mainly includes the defect size information. The weak magnetic signal is sensitive to the stress at the defect, and the signal includes defect size information and stress information at the defect. The ratio factor can reflect the stress at the defect. When the weak magnetic field intensity is low, the average change rate of the ratio factor with the stress at the defect is larger than 9% . With the increase of weak magnetic field intensity, the variation range of ratio factor with stress decreases.