In this paper, the DC loop harmonic resonance of a conventional HVDC system is studied. An analytic calculation method is used to establish the mathematical model of the DC loop harmonic impedance of an LCC HVDC system. Based on the state-space model, an analytical expression of the DC harmonic impedance is derived. The DC loop impedance spectrum is then formed and verified by PSCAD simulation. The coupling relationship between the HVDC parameters and the DC harmonic impedance is analyzed. Finally, a method to suppress broadband resonance of the DC loop by adjusting control parameters is proposed and verified by electromagnetic transient simulation. The results show that the proposed method can effectively suppress the DC resonance of the HVDC system and ensure the secure and stable operation of the system. This work is supported by the National Natural Science Foundation of China (No. 51777079).