Considering the influence of electromagnetic field on the structural deformation, and supposing that the wire’s deformation was small, a natural lateral vibrant control equation of wire carrying electric current under the periodical excitation in the magnetic field was established by chord modeling. The critical condition to chaotic motion of the wire carrying electric current in the magnetic field was researched with Melnikov Method and Galerkin Principle. And factors such as string tension, string distance and current that affected the chaotic motion region were discussed. The results showed that the chaotic motion region of wire carrying electric current increased with the increasing of string tension and string distance respectively, and decreased with the increasing of electric current when the electric current was below a certain value.