The present invention provides a line power control method and system for a unified power flow controller. The line power control method 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 I sedref and I seqref are calculated by means of the outer loop line power control according to line power instructions P ref and Q ref that are input, a measured line power U L , and measured line power P line and Q line ; a converter output voltage reference value U cref 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. The control method is simple and practical, has high reliability, can rapidly and accurately control the line power, and can implement independent decoupling control of active power and reactive power of a line.