The invention relates to magnet control type parallel reactor controllable afterflow method and excitation system. The present method comprises following steps: a controllable afterflow branch constituted by thyristor valve parallel to DC bus two pole is installed at magnet control type parallel reactor excitation system, the cathode of thyristor valve is connected with anode of DC bus, the anodeof thyristor valve is connected with cathode of DC bus;when rectifier is under normal operation state, DC excitation current, rectifier and excitation branch constitutes loop, thyristor valve of controllable afterflow branch does not conduct; when open phase operation or three phase tripping of transmission line occurs, control circuit locks rectifier and triggers pulse and controllable afterflowbranch thyristor valve, controllable afterflow branch conducts and DC exitation current transfers from rectifier to controllable afterflow branch to constitute loop, thyristor valves of rectifier totally shut off. The invention solves the technical problem of magnet control type parallel reactor excitation branch afterflow, which is significant to safety and reliable operation of magnet control type parallel reactor.