The utility model discloses a flexible on -load voltage -regulating transformer of three windings and low voltage network system, the flexible on -load voltage -regulating transformer of winding, including primary winding first winding, secondary winding secondary winding and the secondary winding tertiary winding, wherein the secondary winding capacious in the tertiary winding, does the tertiary winding connect gradually the interchange the direct current alternating -current converter and coupling transformer. Can restrain active distribution network multi -frequency yardstick voltage fluctuation fast. Output voltage can continuously adjustable, has indifference speed -voltage characteristic. Do not have the mechanical action structure, the operation is reliable.