The drive force, the rail-coil cross interaction and the system performance of the rail-coil combined electromagnetic gun have been researched, and an improved structure with four rails distributed symmetrically has been proposed. The theoretical analysis and numerical computation of the structure show that the rail-coil cross interaction brings considerable transverse pressure and motion instability to the normal two-rail combined EM gun, while the four-rail combined EM gun successfully eliminates the negative effect of the cross interaction and obtains much better performance.