A line power control method and system for a unified power flow controller includes outer loop line power control, inner loop valve side current control, and converter valve control. Series-side converter valve side current reference values Isedref and Iseqref are calculated by means of the outer loop line power control according to line power instructions Pref and Qref that are input, a measured line power UL, and measured line power Pline and Qline; a converter output voltage reference value Ucref is calculated by means of the inner valve side current control according to the valve side current reference values that are output by means of the outer loop power control, a measured valve side current, and a measured valve side voltage; and finally, a converter outputs, according to the voltage reference value, a corresponding voltage to control line power to achieve a reference value.