The invention discloses a method of determining capacity of a unified power flow controller on a section of a main network. The method of determining capacity of a unified power flow controller on a section of a main network includes the steps: establishing a data model, accessing an AC power network, of a unified power flow controller UPFC; according to the preset increment gradient, increasing the power on quantity of generators in a sending terminal area; according to the fault that the line exceeds the self thermally-stabilization ration in the AC power network structure when the UPFC doesnot act, determining a first fault set; determining whether the initial fault set is identical to the first fault set, and according to the first determination result, determining the UPFC capacity required by elimination of line overload; determining whether the section power transmission capacity achieves the feasible limitation, and determining the second determination result; determining a UPFC capacity-section power transmission improvement power curve, and calculating the UPFC capacity corresponding to the saturation points in the curve; determining whether the fault of deciding the UPFC capacity in the UPFC capacity-section power transmission improvement power curve changes or not, determining the third determination result, and according to the third determination result, determining the UPFC capacity to be selected; and according to the UPFC capacity to be selected, determining the optimal UPFC capacity.