Abstract: Improper operation in drilling and the process of well repairing will lead to rupture of the casing, fish head (fracture of the casing) missing, which will seriously affect the progress of drilling operations, resulting in huge economic losses. At present, the salvage operation of lost fish is mainly guided by experience judgment, which is less efficient and inaccurate. In order to improve the efficiency and accuracy of fish salvage, this paper uses the principle of weak magnetic detection to locate the position of casing lost fish. Based on the basic theory of electromagnetic field, the point source magnetic field model is established, the attenuation formula of the point source magnetic field with space distance is deduced, and the position of the lost fish is reversed by the magnetic field intensity measured by a high-precision fluxgate sensor. The simulation test of the position of the lost fish shows that the weak magnetic technology can realize the positioning of the lost fish, and the precision of the positioning decreases with the increase of the magnetic source distance. Finally, the established model is applied to the actual project, and the lost fish is found accurately and efficiently.