An overall measuring identification method for calculating the frequency characteristics of the interconnected power system is proposed， which takes full advantage of the merits of the wide area measurement systems（WAMS）， such as the highly accurate and synchronized phasor measurement and the correlativity between frequency and active power to determine the frequency characteristics of the interconnected power system. The correctness of the method proposed is demonstrated from both the mathematical and physical aspects and a simple method of determining the location of perturbation is also presented. By taking North China Power Grid as an example， the perturbation data recorded by WAMS are analyzed and calculated and the results are proved identical with the grid’s actual performance.