The fundamental equations of electromagnetic field were written in the form of duality system,and the variational principle for dual variables was derived for electromagnetic finite element method.To maintain the symplectic conservation conditions of FEM formulation,the integrand was rewritten dymmetrically for duality variables,and the boundary integral terms of variational principle were cancelled mutually for the adjacent elements.The dual variable FEM formulation can avoid the difficulty of C1 continuity condition.The FEM analysis for dual variables was applied to selve the eigenproblems of resonant cavity,and the generalized eigenvalues were solved numerically.In order to eliminate the spurious solutions owing to the violation of zero-divergence requirement in each element,the singular value decomposition (SVD) technique was employed,and the numerical results demonstrated the effectiveness of the method.