After the lysines on the surface of Cu,Zn superoxide dismutase are modified,the enzyme stability increases remarkably.The rearrange of charge structure of the enzyme due to modification results in the change of enzyme electrostatic field.The finite difference solution to the Poisson-Boltzman method is used to calculate the change of electrostatic field and the effect on Cu, Zn ligand structure which play an important role to stabilize the enzyme.The results show the first and second decomposition constants of Cu, Zn ligand complex are reduced by 10³ and 10⁶ after modification respectively.