A new passive approach to reducing the input current's harmonics of a three-phase diode rectifier based on the injection of the third harmonic current is proposed in this paper. In contrast to previous approaches，the star-connected capacitor is adopted to substitute for a transformer as the injection circuit， which operates in the serial resonant state at third harmonic frequency to enhance the circulating harmonic current in amplitude. The principle of harmonic suppression and the implementation of the third injection current are described first. Then， the characteristics of the circuit including the condition for optimal injection， the effects of component parameters' deviation， the load varying range as well as component parameters' error are analyzed in detail. Experimental results show that the new approach has better harmonic suppression performance than passive filtration while simpler and more reliable than active filtration， which gives it broad prospects for future application.