The invention discloses a flexible DC convertor station small interference impedance equivalence method. In small interference analysis of a convertor station at one end of a flexible DC power grid, small interference impedance equivalence needs to be carried out on the rest parts; a flexible DC convertor station control system is complex; and when the small interference impedance equivalence is carried out thereon, multiple factors need to be taken into consideration. The equivalence mathematic model, taking both operating conditions of a primary system and multiple parameters of local control and coordinated control systems into consideration, has higher accuracy, and realizes balance between rapidity and accuracy of small interference analysis.