The invention discloses a pumped storage static frequency converter step-up transformer differential protection amplitude algorithm. Through real-time motor-side voltage frequency measurement and updating, a Fourier calculation point number and a calculation step are determined according to the voltage frequency; according to the Fourier calculation step and a former sampling sequence participating in Fourier operation, a next sampling sequence participating in operation is determined; an interpolation method is adopted to calculate the current value of the next sampling sequence participatingin operation; and according to the current calculation point number, a Fourier formula is adopted to calculate the current amplitude. The algorithm has the beneficial effects of improving the calculation precision and improving the protection response speed.