The weak magnetic stress detection technology supports non?contact online detection of stress damage. It has great application potential in the field of long?distance oil and gas pipeline stress detection technology. However, as the weak magnetic signal is weak and easy to be interfered by the external environment,the detection result is prone to deviation.In order to strengthen the detection ability of weak magnetic signals, based on the microscopic characteristics of ferromagnetic materials, a weak magnetic stress detection model under an external magnetic field was established, and the change law of weak magnetic signal strength with the increase of external magnetic field strength and applied stress was obtained.The excitation characteristics of the external magnetic field to the weak magnetic stress detection signal were described,and a systematic experimental study was conductd.The results show that the tangential peak value and the normal zero point of the weak magnetic signal are located in the center of the stress concentration area, and do not fluctuate with the change of the stress and the external magnetic field.The tangential peak value and the normal peak?peak of the weak magnetic signal increase with the increase of the external magnetic field and stress. As the external magnetic field increases, the excitation effect of the external magnetic field on the weak magnetic stress detection signal increases first and then decreases.