Static Frequency Converter (SFC) is an important equipment of large pumped storage unit starting under pump operating condition. Given the lack of current SFC differential protection, this paper proposes a new protection method, namely amplitude differential protection. As the SFC current at grid side is power-frequency and the current at generator side is variable-frequency, it is difficult to calculate the conventional differential current directly. Based on the basic principle of SFC, the current frequency changes after current flows through SFC, but the effective value is unchanged. Therefore, SFC current RMS (Root Mean Square) at grid side and generator side can be utilized to constitute differential protection scheme. Firstly, the period of variable-frequency current is obtained by using the zero crossing. Then the effective value of variable-frequency current is determined. Finally, an amplitude differential protection method is proposed. In addition, SFC starting model of pumped storage unit is built in Matlab/Simulink and the good quickness and selectivity of amplitude differential protection is verified by the simulation results.