Magnetic signal is one of the main physical field signals of unmanned submarine. Magnetic anomaly information is not only an important signal source of magnetic detectors, but also an important reference information for unmanned submarines in demagnetization design. However, traditional approximate models based on magnetic dipoles are only applicable to the far field, have poor performance in the near field region, and have poor prediction accuracy for targets with certain scales. Therefore, the paper proposes a finite element method for predicting magnetic anomalies in unmanned submarines, and establishes a finite element model of unmanned submarines. The distribution and characteristics of induced magnetic fields in unmanned submarines under constant magnetic fields are simulated and analyzed, which can provide reference for magnetic anomaly prediction and demagnetization design of unmanned submarines. The results show that the finite element method can effectively characterize the distribution information of local magnetic anomalies; Under the external magnetic field, the magnetic induction intensity on the surface of the shell is larger, but the magnetic induction intensity inside the shell is smaller, which is equivalent to the magnetic shielding effect; The geometry of unmanned submarine will affect the spatial magnetic field distribution, and the asymmetry of the shape will destroy the spatial magnetic field distribution. The research in this paper can provide some reference for electromagnetic design, degaussing and magnetic anomaly detection of unmanned submarine, and it can also be used in fields such as marine resource exploration and underwater pipeline monitoring.