Harmonic instability caused by HVDC means that harmonic oscillation doesn’t attenuate or even amplifies seriously when a disturbance happens in the converter station. For decreasing the probability of harmonic instability and guaranteeing the safety and stability of HVDC’s operation， it is necessary to research the harmonic instability deeply. This paper investigates the mechanism of harmonic instability， and enumerates research methods basing on this mechanism. Finally it indicates the methods of trapping harmonic instability， and tables some proposals to the key of researching harmonic instability and control measures in the future.