The invention relates to a double-circuit-line UPFC (unified power flow controller) and a series side converter control method thereof. current decoupling items between double circuit lines are calculated, a decoupling item with current of the other circuit line considered is added to an inner current loop of a series side converter of the double-circuit-line UPFC and has a combined action with current decoupling items of an inner current loop PI controller of the circuit line and dp axis current decoupling items, an output voltage reference value of a converter is obtained and used for controlling line power, power flow decoupling control between double circuit lines can be realized, the interaction during power adjustment of the double circuit lines is reduced, the dynamic response characteristic of a system is improved, and the control structure is simple and easy to implement.