Consecutive commutation failure of HVDC transmission system is an important cause of cascading faults in AC/DC hybrid power system. The distortion of voltage waveform caused by harmonics leads to the reduction of commutation area, which is an important factor leading to the consecutive commutation failure. The existing methods of commutation failure mitigation are mainly aimed at the decrease of amplitude of fundamental frequency voltage and lack of consideration of harmonic effect. To solve this problem, this paper analyzes the influence mechanism of harmonics on the consecutive commutation failure, calculates the influence factors of harmonics on the extinction area, and proposes a quantitative calculation method of the extinction angle setting value considering harmonics. The proposed algorithm increases the setting value of the constant extinction angle controller during commutation failure and maintains a constant extinction area, thus restraining the occurrence of consecutive commutation failure. Finally, the simulation is carried out by electromagnetic transient simulation software PSCAD/EMTDC. The effectiveness of the proposed method is verified by the simulation. This work is supported by Science and Technnology Project of State Grid Shandong Electric Power Company (No. 2017A-58) “Application and Project Demonstration of Global Energy Interconnection based “Source-Grid-Load” Coordinated Control Technology in Large Power Grid Safe and Stable Operation, National Key Research and Development Program of China (No. 2016YFB0900600), and China Postdoctoral Science Foundation (No. 2017M620777).