The invention belongs to the field of electric power system operation control and especially relates to a parameter optimization and setting method for a generator excitation control system. The invention provides a difference adjustment coefficient layering and zoning optimization method of a generator excitation system. The basic idea of the method is: first, generator groups of an electric power system under a power grid company separated layer management system are clustered according to the electrical distance by use of a clustering algorithm, and reactive coherent generator groups are constructed; then, under conditions that it is assumed that difference adjustment coefficients of generator excitation systems in a same reactive coherent generator group are identical, and that equivalent difference adjustment coefficients of the power grid being constant are known, the whole network difference adjustment coefficients are optimized; the difference adjustment coefficient of each generator excitation system is set according to the optimization result. The method makes it more optimized and feasible for the difference adjustment coefficient setting of large numbers of generator excitation systems in the power grid on the basis of considering the convenient management and similar features of generator outputs. The method can be used for difference adjustment coefficient setting of generator excitation systems in power grid operation.