An extended magnetic charge model considering the magneto-mechanical coupling effect and dislocation pinning effect is constructed to improve the effectiveness of the weak magnetic detection method for early damage diagnosis of buried pipelines, and further clarify the quantitative relationship between detection signals and stress and defects. The variations of weak magnetic detection signals caused by various abnormal conditions of pipelines are studied. The effectiveness of the model is evaluated by engineering detection experiments. Research results show that the gradient modulus GM has significant symmetrical peak distortion at the volume pit defect, area crack defect or weld defect. The peak value of the gradient modulus GM increases with the increase of defect size, especially the equivalent depth. The influence of weld misalignment defect on the peak shape and peak value of GM is significant, while the influence of undercut defect is not obvious. Comparing the engineering measured data and theoretical calculation data, the maximum position error is ≤1 m, and the maximum peak error is ≤ 15% , which are both within the acceptable range of engineering detection.