The invention provides a method and device for calculating the effective value of a terminal voltage of a generator, and the method comprises the steps that a rotating speed sensor collects the rotating speed of the generator; a microprocessor calculates the frequency of the terminal voltage according to the rotating speed and the number of pole pairs of the generator, and judges whether the frequency reaches the maximum frequency or not: sets the sampling frequency as fb if the frequency reaches the maximum frequency, or else, sets the sampling frequency as fb*f/fmax; the microprocessor judges whether the sampling frequency meets the Nyquist sampling theorem or not: the microprocessor enables a sampling control signal to a voltage collection module if the sampling frequency meets the Nyquist sampling theorem, and the voltage collection module carries out the sampling of scanning data in a first storage region based on the sampling control signal, and stores the sampled data in a second storage region; the second storage region sets a storage region identification in an effective state; the microprocessor obtains the sampling data when the microprocessor detects that the storage region identification is in the effective state through polling, and calculates the effective value U of the terminal voltage of the generator, so as to achieve the purpose of improving the calculation precision of the effective value of the terminal voltage.