Abstract: Converter transformer is one of the core unit of converter station in UHVDC system. A three-limb four-winding converter transformer model on the basis of the UMEC method is designed, suited for the UHVDC system, to reduce the vulnerability to both winding faults and core vibrations caused by the harmonics and the reactive power deficiency. An electro-magnetic coupling circuit is designed to produce negative equivalent magnetic flux as the current and voltage filtering approaches with extra winding weakening the impact of harmonic propagating between the two original windings. A simulation model is finally developed with a UHVDC system and a designed converter transformer. The harmonic filtering function of the model were tested and validated with excellent performances as theoretical expectation. The results also demonstrate that the electro-magnetic coupling circuit can effectively improve the waveform quality without affecting normal operation of the UHVDC system, which is more convenient for practical engineering applications.