In quantum information theory,von Neumann entropy plays an important role;it is related to quantum channel capacities.Only for a few states can one obtain their entropies.In a continuous variable system,numeric evaluation of entropy is not easy due to infinite dimensions.We develop the perturbation theory for systematically calculating von Neumann entropy of a non-degenerate system as well as a degenerate system. In quantum information theory, von Neumann entropy plays an important role; it is related to quantum channel capacities. Onely for a few states can one obtain their entropies. In a continuous variable system, numeric evaluation of entropy is not easy due to infinite dimensions. We develop the perturbation theory for systematically calculating von Neumann entropy of a non-degenerate system as well as a degenerate system.