This paper proposes a reliability index considering the impact on power gird made by the HVDC transmission system. The application of traditional reliability index of energy availability appears problems and defects gradually. By improving the energy availability from the basic algorithm, the index of operation reliability is designed to evaluate the reliability contribution to overall system, which made by HVDC transmission system. The paper interprets the application of operation reliability by cases, in which the practical HVDC systems in China Southern Power Grid are presented. The result is helpful to provide some reference information on improving the reliability of the HVDC system and making decision from the perspective of optimizing maintenance arrangements.