Abstract: This paper improved current filament method （CFM） which is commonly used in coil gun’s circuit simulation.In the modified CFM,each excitation coil was divided into small sub-coils to improve calculation precision of inductance in equivalent electric network.In order to check the effect of the modification and analyze electromagnetic transient in coil gun launch,the field-circuit method for induction coil gun was introduced.The field model is built based on 3D composite grid method （CGM）,CGM uses two sets of mutual independent grids to discretize solution region,the coarse grids discretize the global region and the fine grids discretize the moving part.In every time step,instead of remeshing it is only necessary to change the coordinates of nodes in fine girds.A three-stage coil gun’s field-circuit model was built,by comparing the results of field-circuit model before and after the improvement of CFM,the validity of modification is proved.