Compared with motor, the rotor structure of electric motor was more complex, which included host and exciter, in result, the accurate calculation of the generator critical speed was highly difficult but indispensable. The rotor critical speed of 4P turbogenerator of 15MW was analyzed and calculated based on the theory of transfer matrix method, and the corresponding equivalent method was explained in detail. The method was verified by actual date and cases, and it was convenient in engineering application.