With the large-scale application of new power electronics technologies such as renewable energy generation, flexible HVDC and STATCOM, the harmonic problem of AC/DC hybrid power grid is becoming more and more prominent. This paper combs the basic concept of harmonic instability of AC/DC hybrid system. The mechanism of complementary resonance and core saturation instability is emphasized, and the solution of the harmonic instability problem is proposed. In addition, the harmonic instability events are investigated comprehensively, and the mechanism and suppression measures of harmonic instability in Niuzhai-Congxi HVDC, Luxi Back to Back HVDC and Yongren-Funing HVDC are analyzed.