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Effective Hamiltonian method is widely used in quantum information.We introduce a method to calculate effective Hamiltonians and give two examples in quantum information to demonstrate the method.We also give a relation between the effective Hamiltonian in the Shr(o)dinger picture and the corresponding effective Hamiltonian in the interaction picture.Finally,we present a relation between our effective Hamiltonian method and the James-Jerke method which is currently used by many authors to calculate effective Hamiltonians in quantum information science.