The invention discloses a weighted aggregation-based modeling method and system of a power system. The method comprises the steps of 1, acquiring a model of each sub-system of a regional power grid to be modeled under the same hierarchy; 2, performing derivation to obtain a corresponding discrete model, and performing analysis to obtain a relation between each model system in the discrete and each component parameter in the regional power grid; 3, judging whether each sub-system discrete model is effective or not according to physical relation characteristic among model coefficients in the discrete model; 4, building a uniform discrete model under the same hierarchy by a weighted aggregation method according to the effective discrete model of each sub-system under the same hierarchy; and 5, performing analysis to obtain the physical relation characteristic among the coefficients of the uniform discrete model built in the step S4 so as to judge whether the uniform discrete model is correct or not, wherein the built uniform discrete model of the regional power grid is effective if the uniform discrete model is correct and can be used for subsequently splicing models. The method can be used for regional power grids in different combinations and with different complexities, and the building of a simulation model and implementation of model splicing are facilitated.