The invention discloses a method for steam feed pump efficiency calculation when a feed pump center tap is opened. The method for the steam feed pump efficiency calculation when the feed pump center tap is opened comprises the steps that the enthalpy values of inlet and outlet fluid and reheat desuperheating water, feed pump inlet entropy, isentropy enthalpy under the pump outlet pressure and isentropy enthalpy under the reheat desuperheating water pressure are worked out respectively; the mass flow rate of feedwater and mass flow rate of the reheat desuperheating water are worked out; the energy absorbed by the flow volume of the reheat desuperheating water and the flow volume of the feed pump outlet under isentropy enthalpy flow under isentropy enthalpy flow and the energy of the feed pump in real flow are worked out; the lost energy is worked out; according to the energy absorbed by the flow volume of the reheat desuperheating water and the energy of the feed pump in real flow and the energy lost calculated by step 3 and step 4, the energy of the feed pump in real flowing and the lost energy, the efficiency of the feed pump when the feed pump center tap is opened is worked out. The method for the steam feed pump efficiency calculation when the feed pump center tap is opened has the advantages that measurement is convenient, a few measuring parameters are needed, and influence on a result by measuring errors is small.