Abstract: Electromagnetic effect generated by explosions has a significant influence on the explosion test. Some metallized explosives even disable the electronic equipment. The mechanism and numerical simulations are investigated to the magnetic disturbance of natural magnetic field observed during the conventional explosion for getting the theoretical model and the properties of the explosion-induced electromagnetic effect. The mechanism of the generated magnetic disturbance is proposed based on a thermodynamic equilibrium ionization model and the magnetic hydrodynamic (MHD) model considering magnetic diffusion. The magnetic disturbance is considered to be induced by the motion of the conductive detonation products in the geomagnetic field. The conductivity, which makes direct influence on the disturbance duration, is described by the ionization model. Combined with the MHD governing equations and the method utilized in explosion issues, two kinds of numerical simulations are performed to investigate the magnetic disturbance during explosions of rectangular explosive in different initial magnetic fields. It is found that the magnetic disturbances are extremely different in value while almost the same in distribution. The distributions of fluid and magnetic parameters are compared at the same time and a conclusion can be obtained that the same distribution of conductivity leads to a similar distribution of magnetic field. The conductivity of detonation products has a huge difference in space distribution and this makes the magnetic field diffused except in the region with high conductivity. Therefore, obvious magnetic disturbance can be only observed in the region of high conductivity. The difference of magnetic field in value is related to both the initial magnetic field and velocity of fluid and can be only extremely observed in the early stage of explosion. A magnetic disturbance induced by the electromagnetic wave generated in the explosion is simulated and compared with the result in reference. The comparison shows a good agreement in the time duration scale of the magnetic disturbance, which proves the reliability of the numerical method used in this paper. The influence of the configuration of explosives in the geomagnetic field on the magnetic disturbance is first considered and the difference in the magnetic disturbance is predicted. The above achievement has not been reported in the previous published investigations. The prediction proposed in this paper needs a further experimental verification and this will be the next stage of our investigation.