Symplectic algorithms and finite element methods(FEM) for Hamiltonian system are summarized,and three conjectures on long-time deviations of computational orbit,energy and symplecticity are proposed.This paper emphasizes the continuous nite element method and its energy conservation,proposes the basic assumption A on H(z) and derives three uniform bounds.It is proved that long-time deviation of nite element orbit grows linearly with time(Fengs conjecture),based on superconvergence result for short time and a new approach.Numerical experiments verify the validity of three conjectures.