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Harmonic instability has greatly influence on the safety and stability of AC/DC transmission system, which had been paid more attention by academic and engineering members. In the paper, Firstly the mechanism of harmonic instability is analyzed, based on which, a new method suppressing the harmonic instability is proposed combing with the technique characteristic of FCC-HVDC. Even harmonic filters are installed at the valve side of new converter transformer in the new method, through suppressing non-characteristic harmonic voltage of AC grid side which is produced by non-characteristic harmonic current of DC side, the positive feedback between non-characteristic harmonic current and non-characteristic harmonic voltage can't generate, then the purpose of suppressing harmonic instability is achieved. Then, referring to CIGRE benchmark testing model, a new testing model is established, in which the Line Commutated Converter (LCC) is replaced by Filter Commutated Converter (FCC), and 2nd and 4th harmonic filter branches are installed in the new testing model. In the last, by comparing the waves of magnetizing current, the core magnetic flux saturation level and the even harmonic current in the prolonged winding switching on and off the 2nd and 4th harmonic filter branches, the simulating result indicates that the new method of suppressing harmonic instability can improve the saturation level of the converter transformer and reducing the even harmonic currents in the prolonged winding. Though, the new method can block the condition of generating harmonic instability, which can greatly decrease the occurring ratio of harmonic instability in DC transmission system.